SHORT PAPERS ON SUBJECTS CONNECTED WITH SCIENCE. No. VI. HEAT, No. 3. EXPANSION. By the term expansion we wish to convey to the reader's mind, that enlargement or increase in the bulk of bodies, which takes place when they are heated; while there is a corresponding contraction when their temperature is diminished. Bodies present themselves to our observation in three states, viz. gases or vapours, liquids, and solids. Common air is an example of a body in the liquid state; and iron a body in the solid state. Gases and vapours expand most, liquids expand less, and solids least of all, when subjected to the same increase of temperature. A knowledge of the rate of expansion of gaseous bodies, is essential to the practical chemist. If 1000 volumes of air (say 1000 cubic inches) were heated from 32 degrees, that is the temperature of ice cold water, to 212 degrees, which is the temperature of boiling water, the 1000 cubic inches, will have become 1325 cubic inches or have increased in size 325 cubic inches. According to Mr. Dalton, the expansion of air, gases, and vapours for 1 degree, is the number used by Chemists in their calculations is a little greater. It is at present the opinion of Chemists, that air, and all gases experience the same increase of volume, by the same increase of temperature; the knowledge of this property of gases, and the amount of their expansion for each degree of heat supplies an easy method for determining what the bulk of a gas would be at a given temperature, provided we know its bulk at any other temperature. The expansion of steam, and the expansion of the vapour of sulphuric aether, have been found to be the same as that of air, it has been concluded from this, that vapours follow the same rate of expansion as gases. 1 483 1 22 · 9 There is no general law of this sort for the expansion of liquids; each liquid having a rate of expansion peculiarly its own, and which we may therefore call specific. If we heat alcohol from 32 degrees to 212 degrees, it will be dilated Oils under the same treatment will dilateth; water ; mercuryth. Liquids differ from gases in another circumstance, their expansion is not uniform; but its rate increases with its temperature; that is, if we add 180 degrees of heat to water at 32 degrees, we raise the 1 but if we add anotemperature to 212, and dilate it 22 ; ther 180 degrees, thereby raising the temperature to 392 degrees, we shall have it dilated more than engineers are aware of this property of the expansion of fluids by heat, it in some measure increases the danger of high pressure engines, in the boilers of which, water is generally heated to about 400 degrees; now at this temperature any increase in its heat produces great expansion. Liquids differ from gases in another remarkable instance; the particles of gaseous bodies repel each other; but those of liquids attract, as is evident from their collecting together in spherical drops; the force of this attraction differs very much in different liquids. It is much greater between the particles of These atmercury, than between the particles of water. tractions (as one would suppose) do diminish, as the distance between the particles of the liquid increase. It is obvious that this attraction between the particles of heated liquids, must act as an antagonist to the expansion produced by heat. Hence the reason that every liquid has an expansion peculiarly its own; and hence also the reason why the rate increases with the temperature; every increase of temperature does and must weaken the attraction between the particles of the liquid, by increasing their distances from each other, and therefore augment the effect produced by a given movement of heat. Water varies in its rate of dilation more than most liquids, its greatest density is a little above 39 degrees, if we heat it above that point, or cool it below it, in either case it expands; this fact was discovered by the Florentine academicians, about the middle of the 17 century; they filled a glass ball (terminating in a long narrow neck) with water, and placed it in a freezing mixture; as soon as the water was cooled down to 39 degrees, its bulk began to increase, consequently it rose higher in the long narrow neck of the vessel, it continued to do this slowly and equally till some portion of it shot into ice, when it sprung up with the greatest velocity; at first it was doubted if this effect was not produced by a contraction of the vessel in which the water was contained, careful experiments proved that this was not the case, and established the fact that the greatest density of water was at about 39 degrees; at that it expands or increases in volume, when heated above or cooled below that point; the benefit which mankind derive from this peculiar property of water, we shall perhaps point out in our next paper. S. THE DAHLIA. Oh much do I love thee, varied flower, Can a welcome true ensure thee. Some say thou dost too boldly shew To the passing eye, thy face: But I thank thee that when the roses are dead Thou dost well supply their place. Thou thus art like the manly heart On which the spirit confides; When other pleasures have proved untrue, And if the first touch of the winter's cold Still like that heart thou seemest to me, In a land which affections cherish! Oh much do I love thee, varied flower, FALL OF THE LEAF. [From DRUMMOND'S Botany.] THE leaves of trees and shrubs, having lasted the time allotted them by nature, shrink and drop off, producing annually the phenomenon called the fall of the leaf.' With respect to the cause of this, it is sufficient at present to say, that each leaf falls because it is weakened or dead, and also because it is separated by an action in the living branch from which it grew. The bare death of a leaf is not sufficient to cause its fall; for when both leaf and plant are killed by lightning, by a cutting wind, or by any other sudden cause, the dead leaf will adhere tenaciously to the dead branch. There are some plants which it is very difficult to preserve by drying in the usual manner, because their leaves all separate; and for this reason, they become dead sooner than the parts from which they grow, and these parts retain vital action enough to throw them off. The remedy, however, is simple; imitate the stroke of lightning, or whatever will at once kill the whole plant: all will then die together, and consequently the dead branch will have no power to cast off the dead leaf. The remedy is to dip the specimen in boiling water before committing it to paper. The heaths, especially, are said to require this treatment, as do also the succulent plants, though on a somewhat different account; for as the latter can live almost independent of roots, they continue to vegetate during the usual process of drying, and it is not uncommon to find them, when pressed in books, running through the whole process of flowering, and even producing seeds. Brown mentions that the leaves of the smooth acacia (which are not succulent indeed, but sensible like those of the sensitive plant) will spread and contract, after they have been in paper for a month or six weeks. The change of temperature from hot to cold seems to be one principal circumstance connected with the death and fall of the leaf; and hence it is, that European trees grown in the southern hemisphere, cast their leaves at the approach of winter there, which is about the same period of the year that they put them forth in their own climate. Some birds cast their feathers all at once, and in consequence, being unable to fly, are caught in great numbers. These may be compared to most trees in our regions, which part with their leaves in a few weeks, and remain bare till the following spring. But in most birds the casting and renewing of their feathers is a gradual process; and when the change is going on, no inconvenience from want of clothing is felt by the animal; as although it is constantly |