during the sleep of his physical body, while he himself is clad in his astral body in full consciousness on the astral plane. He will also have been taught while thus functioning in the astral world to bring help, instruction and comfort to the inhabitants of that world, who having laid aside their physical bodies at the gateway of death have passed beyond the physical plane. This phase of occult work is sometimes called that of the Invisible Helpers and is performed, be it understood, by men still having physical bodies and who have developed this power of functioning consciously outside the same on inner planes. The pupil will further have been trained in mediation, and this effective practice both in waking consciousness and outside the physical body during its sleep will have quickened and brought into active exercise many of the higher powers.

When the pupil has developed the fifth qualification of the probationary path he is ready for initiation upon the part proper, henceforth to serve his Master, whom he now meets face to face, in helping forward the evolution of the race, his life, "to be offered up on the altar of humanity, a glad sacrifice of all he is, to be used for the common good." This path consists of four distinct stages and the entrance to each is guarded by an initiation. Ere the second initiation can be passed the pupil must lose the sense that the separated, personal self is a reality, and must feel himself one with all; he must destroy doubt and superstition by knowledge; ere he passes the third he must bring into full working order the inner faculties, those belonging to the subtler bodies; now he needs to incarnate but once again; ere the fourth is passed he rids himself of desire and aversion and sees the One Self in all. At this stage he needs to return no more. The fourth initiation admits him to the last stage of the path where he throws off all clinging to life in form and all longing for even formless life. Then he casts off the "I-making" facultypride, irritability and ignorance, and henceforth dwells on the plane of unity. The man is then perfect, is free, the liberated one. He has won Nirvana. "He has completed man's ascent, he touches the limit of humanity; above him there stretches hosts of Mighty Beings, but they are superhuman; the crucifixion in flesh is over, the hour of liberation has struck and the triumphant "It is Finished' rings from the conqueror's lips, he has vanished into light nirvanic. But being now Master of Compassion as well as Master of Wisdom, he returns from that light to earth, henceforth to devote himself to the service of humanity with mightier forces at his command than he wielded while he trod the path of discipleship, bending all his sublime powers to the quickening of the evolution of the world. Such an one was the Buddha - such the Christ and such the few Great Souls who tread the earth to-day, secluded from its external strife, yet pouring down upon mankind from the great heights of their sublime advancement inestimable blessings, guiding by means of their divine powers whole races and nations, but unknown to all but the few earnest souls who come to them through the ancient gateway of Occultism, the portal of which has throughout all time stood ajar for the resolute pilgrim."

Before the cycle of time shall close and all manifestation cease, the greater portion of man

kind will have reached this high stage of growth. And then shall all be gathered unto Him for the great Cosmic rest only after æons and æons of time, to emerge again with Him, to be the Architects and Builders of future universes. Such are a few of the teachings of the Ancient Wisdom, given forth at this time as Theosophy and Occultism.

Bibliography. Anderson, 'Reincarnation' (San Francisco 1896); id., Septennary Man' (ib. 1895); Blavatsky, H. P., Isis Unveiled' (2d ed., 2 vols., Point Loma, Cal., 1910); "The Key to Theosophy) (3d ed., 1913); The Voice of the Silence'; 'First Steps in Occultism'; 'The Secret Doctrine' (2 vols., Point Loma, Cal., 1909); Besant, Annie, The Ancient Wisdom' (new ed., London 1910); The Seven Principles of Man) (new ed., New York 1904); Walker, E. D., Reincarnation' (New York 1904); id., Karma' (ib. 1905); Death and After) (London 1901); Man and His Bodies'; 'The Building of the Kosmos'; 'The Self and Its Sheaths'; 'Man, Whence and Whither) (Madras 1913); Evolution of Life and Form'; The Birth and Evolution of the Soul'; A Study in Consciousness'; 'Four Great Religions'; 'The Religious Problem in India? Thought Power: Its Control and Culture'; Theosophy and the New Psychology) (rev. ed., London 1909); Esoteric Christianity) (New York 1910); Avataras'; 'In the Outer Court'; 'The Path of Discipleship'; 'Ancient Ideals in Modern Life'; Hartmann, Franz, 'Magic Black and White' (7th ed., New York 1904); Ingalese, Richard, 'From Incarnation_to Incarnation (rev. ed., id. 1908); Fussel, J. H., 'Incidents in the History of the Theosophical Movement (Point Loma 1910); 'Bhagavad Gita' (trans.); Sinnett, A. P., (Esoteric Buddhism (Boston 1884); Growth of the Soul' (Boston 1885); 'Occult World' (London 1896); Mead, G. R. S., 'Fragments of a Faith Forgotten'; 'Orpheus'; Pistis Sofia'; 'Simon Magus'; Leadbeater, C. W., An Outline of Theosophy; The Astral Plane'; 'The Devachanic Plane'; 'Clairvoyance'; 'Invisible Helpers'; 'Dreams'; 'Man Visible and Invisible (illustrated); The Christian Creed' "The Other Side of Death'; 'Some Glimpses of Occultism'; Collins, Mabel, 'Light on the Path'; Judge, W. Q., Ocean of Theosophy' (Los Angeles 1915); id., 'Echoes from the Orient (1910); Scott-Eliott, W., Story of Atlantis'; Williamson, William, The Great Law'; Olcott, H. S., Theosophy, Religion and Occult Science'; 'Old Diary Leaves' (Vols. I, II, III); Cooper-Oakley, Mrs., Traces of a Hidden Tradition,' etc.; Hinton, C. H., "The Fourth Dimension'; Edgar, Lillian, 'Elements of Theosophy'; Lillie, A., Madame Blavatsky and her "Theosophy" (London 1895); Olcott, H. S., Old Diary Leaves' (3d series, New York 1895-1907).

The materials for the above sketch have been drawn more or less literally from the above-mentioned works.

A. P. WARRINGTON,

Fellow of the Theosophical Society. THEOTOKOS, a title given to the Virgin Mary by a decree of the Third Ecumenical Council, which met at Ephesus 431 to condemn the Nestorian heresy. The term means "bringing forth God" and is intended to formulate the

fact implied in the annunciation of the angel to Mary that holy thing which shall be born. of thee shall be called the son of God." Theotokos (Latin, Deipara) implies that the child of Mary was of divine nature from the moment of his conception; that he was not born mere man, to whom, as Nestorius taught, the divine nature was subsequently imparted.

THERA, tha'ra or the'ra, an island in the Egean Sea, belonging to the Grecian Archipelago, the most southerly of the Cyclades. It is of volcanic formation and presents many interesting phases. New islands are formed from time to time, by its volcanic action, notably the last in 1866. Scientists have given the island considerable attention and many observations have been made and records kept. The soil is very fertile.

THERAMENES, the-răm'ě-nēz, Athenian politician of the 5th century B.C. He first became prominent in 411 as a member of the Council of 400, but perceiving the inevitable downfall of that government he joined the opposing party and assisted in its overthrow. In 410 B.C. he was in command of a portion of the Athenian fleet, with which he cruised about and exacted tribute from the neighboring islands, and later he joined the fleet under Thrasybulus, with whom he took part in the battle of Cyzicus. He subsequently served with Alcibiades, participated in the siege of Chalcedon in 408 B.C. and in the capture of Byzantium. After the battle of Arginusæ, in 406, he repaired to Athens, and in order to exculpate himself for not saving the crews of the ships accused his colleagues of neglect, and through his testimony they were sentenced to death. In 404 B.C., when Athens was besieged by the Spartan general Lysander, Theramenes was sent as an envoy to negotiate terms with Sparta. He purposely remained on his mission for three months, during which time Athens was reduced to dire extremity and was forced to make peace upon such terms as to place her at the mercy of the Lacedæmonians. He was elected one of the Thirty Tyrants (q.v.) after the peace, and at first heartily supported the measures of the government. The violent

measures of Critias and his colleagues, however, induced him to form an opposing party, which rapidly increased in strength. He was accused by Critias of being a public enemy, dragged to prison and forced to drink the hemlock.

THERAPEUTICS, that branch of medicine which deals with the treatment of disease. Natures cures; physicians treat. This is an old dogma as true to-day as it was when uttered over 2,000 years ago by a famous Greek physician. Scientists have termed that power of the human body the vis medicatrix naturæ, by means of which nature tends to right her own disordered organs, and it is important to recognize this all-important function of the body, and not to interfere by overzealousness in the use of potent agents with nature's own slowly acquired powers of self-restoration. An innocuous and half-hearted therapy, however, is not the logical system, even if such reconstructive powers of nature are granted. The modern conception of disease does not admit of ready characterization. In the articles on disease and pathology (q.v.) it is attempted to define what

is meant by disease, but such definitions are far from being satisfactory because of the immense variety of diseased conditions, their manifold causation and their devious progressions. In all disease certain groups of physiological functions are altered, and in the attempt to aid nature to bring these disturbed physiological activities to a normal condition the chief work of the therapeutist lies. It makes little or no difference what the agencies may be that are used—often, as has been said, none are necessary- yet the ideal of the therapeutic art is to restore to normal condition, as far and as quickly as possible, the disturbed bodily functions. While it is true that the physician of to-day employs many of the agents used by physicians thousands of years ago, there is a far greater precision and a much more comprehensive series of ideas concerned in their use by the well-equipped modern therapeutist than was possible to his early predecessors. The one aim of the physician throughout the centuries has been the alleviation of suffering and the restoration of the diseased to health or to comfort; and notwithstanding the many small cliques and divergences of opinion, the progress toward scientific therapeutics has been steady and increasingly satisfactory. At the outset of this summary of therapeutics it may be well to consider for a moment the different classes of agents at hand by the use of which, singly or combined, the physiological functions of the body may be modified, and thus be made useful in the healing art.

Expressed categorically, the different therapeutic agents or systems are: (1) Suggestiontherapy; (2) Dietotherapy; (3) Physicotherapy; (4) Mechanotherapy; (5) Pharmacotherapy; (6) Surgicotherapy. Each of these will be briefly discussed.

tem.

Unquestionably the oldest and yet strongest therapeutic agent is suggestion. The power to heal by faith is not the special property of any sect or class, nor the exclusive right of any sysBelief in gods and goddesses, prayer to idols of wood, of stone, of gossamer fiction, faith in the doctor, belief in ourselves engendered from within or from without - these are all expressions of the great therapeutic value for healing that resides in the influence of mental states on bodily functions. These will not move mountains; they cannot cure consumption; they do not influence a broken leg, nor an organic paralysis; but suggestion, in its various forms, may be and is one of the strongest aids to all therapeutic measures. Of its abuse by designing hypnotists, blackmailers, clairvoyants and a motley crew of parasites, space does not permit particularization. The human mind is credulous- it believes what it wants or wills to believe; and the use of suggestion in therapeutics is one of great power for good and for evil.

The treatment of disease by diet constitutes another large branch of the therapeutic art. Modern chemistry has resolved all foods and drinks into their elements, and has given the physician valuable aids in the treatment of a large number of diseases. Many obscure disorders, such as diabetes, gout, myxedema, obesity, etc., are closely allied with certain defects in the metabolism of certain physiological systems of the body. Many of these are best treated by

a dietary regimen, combined, it may be, with other means. Many of the minor disorders of the digestive tract (see DIGESTION) are best treated by regulation of the diet.

By physicotherapy is meant the use of certain physical agents such as heat, cold, light, electricity, etc. Certain forms of application of the agents are termed (a) hydrotherapy (q.v.), in which heat and cold are applied by means of water; (b) phototherapy (q.v.), in which light is used in the treatment of disease. Sunlight, electric light, ultra-violet rays (Finsen's light), X-rays, radio-active substances these all exert

on the tissues of the body certain influences that, properly applied, may bring about restoration of disordered functions, and thus aid nature in curing disease.

Mechanotherapy is the use of mechanical movements in treatment. Massage, vibration, gymnastics, Swedish movements, etc., are some of the different forms in use. Under the name of osteopathy (q.v.) it is attempted to elevate one of the oldest aids to treatment, in use by the Greeks, Chinese, etc., to the rank of an exclusive system.

Under the terms pharmacotherapy and surgicotherapy are included the treatment of disease by drugs, so-called, and by surgical means.

So

far as treatment by means of drugs is concerned, it is interesting to note that practically all drugs act on the tissues of the body in some chemical or physicochemical manner. They may have a certain selective action on certain tissues of the body. Thus the large group of the alcohols, comprising ethers, aldehydes, chloroform, hypnotics such as trional, sulphonal, veronal, urethane and a large number of others - have a selective action on the nervous tissues of the brain, benumbing their activities and causing drunkenness or anæsthesia or sleep, according to the dose or other conditions. Others act on sensory nerves, diminishing pain; such are cocaine, opium, antipyrin, acetanilid, phenacetin, cannabis, etc. Again, other remedies are chiefly on the intestines, giving the large group of cathartics, and so the entire list of drugs might be analyzed. Given a knowledge of the selective or generalized action of the drugs, the power that they may exert in correcting abnormal physiological activities is solely a matter of application and in accordance with the correct interpretation of the cause of the disturbed functions will the therapeutic application be of direct value or not. In other words, drugs are nothing more than chemical agents which may be used to modify certain physiological activities; if by their well-known power in modifying these activities other morbid activities may be corrected, they are agents for good. For a consideration of surgicotherapy, see SURGERY, HISTORY OF GENERAL.

One other phase of therapeutics remains to be considered. It has been pointed out that nature has resources of her own for overcoming certain types of disease. Can these natural powers be so played upon or affected as to increase their operation? Along this line new paths have been opened up since the recognition of a large class of diseases known as infectious. Bacteriology (q.v.) has taught that disease organisms (bacteria) cause the disturbance (disease) in the body not so much by their physical presence as by the extremely virulent

poisons that they form. It is the struggle of the human body to get rid both of bacteria and poisons that makes the disturbance which is called the disease. Thus the diarrhoea of typhoid is one of nature's efforts to throw off poisons; the high temperature and crisis of pneumonia is a supreme effort of the body, which sometimes succumbs under it, to deal a death-blow to the pneumococcus, the microcrganism that causes the disease. The high temperature in malaria (q.v.) unquestionably kills off a great many of the parasites in the blood.

But in addition to these larger and more palpable efforts on the part of nature to overcome the invader, a series of subtle and intricate defenses are at work in the blood-serum, some of the elements of which are known. Thus in some diseases there is elaborated in the blood-serum a direct chemical antidote, an anti-toxin, to the poisons of the invading bacteria. Such a protective power is found in diphtheria (q.v.). (See ANTITOXIN). Vaccination, the discovery of which was almost the result of an accident, is a form of serum-treatment. (See SERUM-THERAPY). The details of the reasons why immunity is conferred by the modified smallpox of the cow are not yet known, as the true cause of smallpox is not indubitably proven, but the time is not far off when the different factors herein involved will be unearthed. A large number of other questions are concerned in this great question of natural and acquired immunity (q.v.).

Human progress has been likened to the advance of a drunken man, and it is certain that the ups and downs and side-lurches and backward steps in the path of therapeutics have been many; but withal there has been a broad blaze of progress. That which has been tried and been found to be true has been grasped and has become the heritage of the whole community. The physician class have shared with the people at large in the general movements; there have been many side-tracks from the broad road of therapeutics; innumerable pathies have had their little day or their 100 years; each in its turn has contributed what kernel of truth it possessed to the progress of the mass. Bibliography.--Bernheim, Suggestive Therapeutics Dietetics'; Hutchinson-Thompson, Physicotherapy, Mechanotherapy'; Cohen, 'System of Physiologic Therapeutics Pharmacotherapy; Brunton, The Action of Medicines'; Cushny, A. R., Pharmacology and Therapeutics (6th ed., Philadelphia 1915); Forchheimer, F., "Therapeusis of Internal Diseases' (2d ed., 5 vols., New York 1914); Hare, H. A., Modern Treatment' (2 vols., Philadelphia 1911); id., 'Textbook of Practical Therapeutics (15th ed., ib. 1914); Schmiedeberg, Arzneimittellehre'; 'Surgery); MikuliczBruns, edited by W. T. Bull, 'Immunity); Roger, Infectious Diseases'; Ehrlich, (Seitenkettetheorie'; Vaughan and Novy, 'Cellular Toxins.'

(

SMITH ELY JELLIFFE, M.D., Editor of Journal of Nervous and Mental Diseases.

THERAPIA, a health resort on the Bosporus, nine miles north-northeast of Constantinople, of which city it is a suburb. It is the ancient Pharmacia ("drug-cure") where tradi

tion says Medea spread her drugs. One of the earlier patriarchs of Constantinople_changed the name to Therapia ("cure"). It marks the limit between what is locally called the Upper Bosporus and the Lower Bosporus. Here are the summer residences of European ambassadors, Turkish officials and wealthy Levantines.

THERESA, Saint. See TERESA, Saint.

THERESIOPEL, tĕr-a'zē-o-pěl, or MARIA-THERESIOPEL, mä re'ä-tĕr-a'zē-o-pěl (Hung. Szabadka), Hungary, a free town in the county of Bacs-Bodrog, 25 miles southwest of Szegedin, in a broad plain between the Danube and the Theiss. The buildings worthy of notice are the churches of Saint Theresa, church of the Franciscans, a beautiful Greek church, town-house, etc., theatre, barracks, gymnasium and music-school. The industrial works include tanneries, linen and leather manufactories and dyeworks. Wheat, tobacco, wine and fruits are grown and cattle-raising is important. There is a brisk trade in horses, cattle, sheep, hogs, hides and wool. There is a fashionable watering-place in the vicinity. Pop. 94,610.

THERIAC, or THERIACA, a compound said to have been first prepared by Andromachus of Crete, who was physician to the Emperor Nero. It was supposed to be an antidote to poison, and continued in use throughout the Middle Ages. As prepared in Venice and other places it was a compound of 64 drugs, pulverized and reduced by means of honey to an electuary.

THERIODONTIA, a sub-order of the Theromorpha (q.v.), large extinct carnivorous reptiles, with shortened coracoid and doubleheaded ribs.

THERMÆ, ther'me (from the Greek thermai, signifying originally warm or hot springs), properly warm baths, but also applied generally to the baths of the ancients. During the Roman Empire the buildings for this purpose were constructed with great splendor and adorned with paintings, statuary, libraries, gymnasia and public walks. The baths of Nero, Titus, Caracalla and Diocletian at Rome were distinguished for the magnificence and luxury displayed in their construction. See BATH HOUSES.

THERMAL SPRINGS. See SPRING. THERMIC FEVER. See SUNSTROKE. THERMIDOR, ther-mi-dor' (Fr. ter-medor), the 11th month of the year in the calendar of the first French republic. It commenced on 19 July and ended on 17 August. See CALENDAR.

THERMIT, a name given by Goldschmitt to a mixture of fine aluminum filings or powder and iron oxide. When this mixture is ignited by some suitable means the aluminum unites vigorously with the oxygen of the iron oxide, forming a very pure variety of steel and a slag consisting mostly of aluminum oxide. This union of finely divided aluminum with oxygen gives a very intense heat, about 3,000° C. Besides the ordinary thermit (iron oxide and aluminum filings) other mixtures may be prepared from aluminum and the oxides of nickel, cobalt, chromium, manganese, etc.

When these are ignited in a properly prepared crucible violent reaction takes place, the oxygen of the oxide being taken up by the aluminum, leaving a very pure metal, nickel, cobalt, etc. This process is now much used to get metals from those oxides that heretofore have resisted all ordinary methods of reduction. When ordinary thermit is ignited the temperature produced is so high that the iron and the slag are left in a molten and highly-heated condition. If this iron is allowed to flow on to another piece of iron or steel it will heat it enought to soften it and the whole will harden to a homogeneous mass. In this way it can be used to replace broken parts of machinery, to mend broken or cracked propeller shafts, to weld together railroad rails so as to form one continuous rail, etc. Some of the important features in this process are its cheapness, ease of execution and the fact that machinery, etc., can be repaired in position. The method is to surround the part to be repaired with an ordinary mold box; a magnesia-lined crucible with a plug in the bottom is placed over the opening; the thermit is placed in the crucible, ignited and as soon as the violent reaction has subsided the plug is pulled and the white hot metal allowed to flow into the mold. Iron tubes can be welded together by placing the ends in a mold and allowing the thermit product to flow in in such a way that the liquid slag first comes in contact with the tubes. The slag forms a protective covering which prevents the hot iron from uniting with them, though it does allow the tube ends to become hot enough to unite as one piece. Railway rails can be joined where they lie on the track, and large pieces of broken machinery in mines or on shipboard can be readily repaired without removal.

THERMO BAROMETER.

MOMETER.

See THER

THERMO-CHEMISTRY, or THERMAL CHEMISTRY, that branch of physical chemistry which deals with the thermal changes which occur when chemical reactions take place or when a body or system (such as a solution) undergoes certain kinds of physical change. Its precise limits are not easy to define, since the subject merges into ordinary chemistry on the one hand and into thermodynamics on the other. Any chemical operation can be considered from two points of view, according as we are interested in the modification that it produces in the nature of the substances that are involved or in the quantity of energy which is absorbed, liberated or otherwise transformed at the same time. It is the province of thermochemistry to investigate the transformations of energy that occur in such cases. The complete discussion of the energy-transformations that accompany a given chemical change should include the consideration of every type or form of energy which may be present; but the investigations which have hitherto been made have related chiefly to the quantities of heat which are liberated or absorbed, and it is to this circumstance that the science owes its present_name, "thermo-chemistry."

The quantity of heat that is liberated or absorbed during a proposed chemical reaction can be determined by causing the given reaction to take place in the interior of a calorimeter. The

particular form of calorimeter that is to be used will naturally depend to a considerable extent upon the nature of the reaction that is to be studied. If the problem consists in the determination of the quantity of heat that is liberated when two given liquids are mixed, the calorimeter commonly consists of a platinum vessel, capable of containing from 500 to 1,000 cubic centimeters, placed inside of another vessel of silver; the space between the two vessels being filled with water. The liquids that are to be examined are brought to the same temperature as nearly as possible, and are then mixed in the platinum vessel. The rise of temperature of the calorimeter being noted, and the masses and specific heats of the various parts of the calorimeter (and its contents) being determined by separate experiments, we are then in position to calculate the quantity of heat energy liberated by the reaction. For detailed information with regard to the various kinds of calorimeters that are used, and for a discussion of the sources of error to which such instruments are liable, reference must be made to extended works upon heat and thermo-chemistry. (See the references at the end of this article).

In thermo-chemical work, the unit of mass is almost invariably the gram; and the gram is always understood, when no other unit is specifically mentioned. The unit of heat is also understood to be the calorie, which, for thermo-chemical purposes, is defined as the quantity of heat required to raise the temperature of one gram of water by one Centigrade degree, when the temperature of the water is in the vicinity of 18° or 20° C. The notation that is employed in expressing the results of a thermo-chemical experiment upon the heat that is developed by a given chemical reaction is simple. The formulæ of the substances that react are written within square brackets, and separated by a comma or a colon; it being understood that the number of grams that are present of any one substance is equal to the molecular weight of that substance. A sign of equality is written after the bracketed formulæ, and on the right hand side of this sign the number of calories of heat generated or absorbed by the reaction is written; a positive sign being prefixed (or suffixed) when heat is evolved, and a negative sign when it is absorbed. The indices that are attached to the symbols of the various elements are written above those symbols, instead of below. For example,

[H',O]+68,360

signifies that when two grams of hydrogen and 16 grams of oxygen, both at about 18° C. and under ordinary atmospheric pressure, combine to produce 18 grams of water (also at 18° C.), the quantity of heat that is evolved is sufficient to raise the temperature of 68,360 grams of water by one Centigrade degree; the temperature of the water being about 18° C. When a compound is broken up into its constitutent parts, the bracketed formulæ are preceded by a negative sign. Thus the expression,

- [H,CI]=-22,000

signifies that when, by any means, 36.5 grams of hydrochloric acid are decomposed so as to set free 1 gram of hydrogen and 35.5 grams of

chlorine, the change is accompanied by the absorption of a quantity of heat that would be sufficient to raise the temperature of 22,000 grams of water by 1° C.

The "heat of formation" of a substance is the quantity of heat given out when the substance is formed from its constituents; it being taken as negative when the formation of the substance is accompanied by the absorption of heat. In general, any given substance may be prepared in various ways, from different materials or constituents; and in such cases the heat of formation will be different, according to the particular substances that are regarded as constituents. For example, sulphuric acid might be prepared from sulphur, oxygen and water, according to the equation

S+30 + H2O = H2SO1,

or from sulphur dioxide, oxygen and water, according to the formula

SO2+O+H2O= H2SO1,

or from sulphur trioxide and water, according to the equation

SO3 + H2O= H2SO.;

and the heat of formation will be different in these several cases, if we regard the immediate materials from which the acid is prepared as the "constituents" of the acid. But if the acid is prepared from the elements sulphur, hydrogen and oxygen (all three being taken in certain standard initial physical states in each instance), then the heat of formation will be the same, whether these elements combine directly to form the acid, or whether the oxides of sulphur and hydrogen are first produced. and these subsequently combine to form the acid. In fact, the principle of the conservation of energy shows that when a body or a system of bodies passes through a succession of changes, either physical or chemical in nature, so as to pass from one given initial state to another given final state, the total change in the internal energy of the body or system is identically the same, whatever the nature of the transformations may be, by which the change is effected. Hence it follows that whenever the heat that is emitted or absorbed in the course of a reaction represents nothing but the decrease or increase in the internal energy of the system, the total quantity of heat that is emitted or absorbed will be entirely independent of the way in which the transformation takes place. If, however, the system performs external mechanical work in passing from its initial state to its final state, the heat energy that is actually given off will be less than that corresponding to the change in the internal energy by the amount which is expended in performing the external work. In many of the reactions that are considered in thermo-chemistry, the external work that is performed is too small to be of any material significance, and in these cases the heat energy that is emitted or absorbed in the passage from one state to another may be logically regarded as independent of the kind of transformation that the transition involves. some cases, however, and particularly when the systems under consideration are partly or wholly gaseous, the external work that is performed is great enough to require serious con

In