2 of the composition P ̧N,O,H2Age and P,N,O,H,Ag respectively.' These are salts of the open-chain acids. A similar salt is obtained by precipitating an ammoniacal solution of pentametaphosphimic acid by silver nitrate, but in accordance with its less acid properties the quantity of silver does not reach 10 atoms, being in the specimen analyzed only 8.76 atoms, but the figures correspond closely to a derivative of amidotėtrimidopentaphosphoric acid, P5N5OH12, rather than pentametaphosphimic acid. Dried at 100° it gave: It appears, therefore, that in alkaline solution the metaphosphimic acids become open-chain acids, just as lactones and lactams give - and d-oxy- and amido-acids. On drying at 100°, the yellow silver salts become gray without loss of weight, probably owing to separation of silver oxide. This is perhaps due to a tendency to revert to the lactam form, with separation of silver oxide, rather than water. DECOMPOSITION OF PENTAMETAPHOSPHIMIC ACID. Under Pentametaphosphimic acid is markedly more stable in acid solution than trimetaphosphimic acid. In the section on the latter I described in detail its decomposition products when acted on by nitric acid. identical conditions the rate of decomposition of pentametaphosphimic acid was found to be very much slower. On account of this greater stability the action of hot acetic acid gives more satisfactory results. No attempt was made to isolate all the products, as these are numerous and the analytical difficulties considerable. The following were identified: Tetrametaphosphimic acid, P,N4O×H ̧. Five parts sodium pentametaphosphimate are dissolved in 15 parts water, 5 parts strong acetic acid added, and the solution heated in the water bath. The separation of a crystalline precipitate of acid sodium tetrametaphosphimate begins in about fifteen minutes and continues for perhaps eight hours, at the end of which time the solution gives no precipitate with magnesium chloride, indicating the absence of pentametaphosphimate. The sodium tetrametaphosphimate, being insoluble in a solution of sodium salts, is completely precipitated. It is filtered off and washed a little with saturated sodium acetate solution. The In the third section of this paper the formula P4N4O, Ag, was ascribed to the per-silver salt of tetrametaphosphimic acid; a recalculation of the analyses in the above manner, however, shows that they correspond much more closely to the open form P1NO,H,Ags, with a slight deficiency of silver. filtrate is concentrated to about one-half with addition of about 5 parts solid sodium acetate, whereby sodium triimidotetraphosphate separates, in the form of flat prisms or plates, which are washed with a little saturated sodium acetate solution and purified by dissolving in water and precipitating by alcohol, or by adding solid sodium acetate to their hot solution. On further concentrating the filtrate, large plates are often obtained, which give, with silver nitrate and nitric acid, a crystalline precipitate of the characteristic silver diimidotriphosphate, P ̧Ñ2OH1Agз. No satisfactory and certain method can be given for the separation of these two acids. The sodium tetrametaphosphimate thus obtained usually has the form of spindles, but, when well developed, consists of brilliant double pyramids. It had not been observed at the time of writing the paper on tetrametaphosphimic acid. It is obviously an acid sodium salt, as it is formed only in the presence of acetic acid or a limited amount of a stronger acid. It was also obtained directly from the original sample of the acid, but was not analyzed, as it is difficult to obtain a sufficient quantity uncontaminated by the free acid. It is moderately soluble in water, but almost insoluble in solutions of sodium salts and is highly characteristic of this acid. The crude salt was dissolved in ammonia, with the addition of some ammonium nitrate, and precipitated by nitric acid as PNO8H6(NH4)2. The precipitation is almost quantitative. This pure salt was dissolved in ammonia, largely diluted, and, after acidifying with nitric acid, precipitated by an excess of silver nitrate in the form of the characteristic P,N,O,H,Ag. This gave: The free acid was obtained by decomposing the silver salt with hydrochloric acid in the characteristic form of difficultly soluble needles. It gave: A comparison of the acid potassium, sodium, and ammonium salts and neutral ammonium salt with those prepared from a sample of tetrametaphosphimic acid from tetraphosphonitrilic chloride showed them to be identical in form. The yield of tetrametaphosphimic acid appears to be nearly independent of proportions and concentration, and was about 12 per cent of that required by the equation P6N6O10H10+ 2H2O=P4N408H8+NH1H2PO4. Triimidotetraphosphoric acid, PO(OH)2.NH.POOH.NH.POOH.NH. PO(OH)2.-The sodium salt of this, the third member of the imidophosphoric acid series obtained, crystallizes well in small rhombic or sixsided plates. The several specimens obtained were not analyzed, but converted into the silver salt. By analogy with PзN2O:H7, there can be little doubt that it is the acid salt, P4N3O10H5Na. It is easily soluble in water, but almost insoluble in saturated sodium acetate solution or dilute alcohol. It gives a precipitate with magnesium nitrate only on adding ammonia and ammonium chloride, when a voluminous precipitate is obtained, which, on standing, slowly changes to minute crystalline spherules. Silver triimidotetraphosphate, P,N3O10H5Ag1, is obtained as a voluminous amorphous white precipitate by adding silver nitrate to a solution of the sodium salt faintly acidified with nitric acid. On long standing under its mother-liquor it becomes crystalline and this change occurs in a few moments on washing the precipitate, shrinking to a heavy, sandy, crystalline powder, a very characteristic behavior. The same change can be observed under the microscope in the partially washed salt, the amorphous portion changing to minute particles showing active Brownian movement, which aggregate to prisms which are long and pointed and often tufted at the ends, and which can not be distinguished in appearance from silver tetrametaphosphimate. From a decidedly acid solution the salt is deposited slowly and without the intermediate amorphous form. Triimidotetraphosphoric acid could not be obtained from tetrametaphosphimic acid, being much less stable than the latter. It is therefore probably derived from the breaking down of the chain of phosphorus and nitrogen atoms formed by the hydrolysis of the pentametaphosphimic ring. It is obtained as sodium salt by saponifying hexaphosphonitrilic chlo ride in ethereal solution by sodium hydroxide in the manner above described. In neutral or acid solution it has the lactam form, as shown by the composition of the silver salt, but in alkaline solution it has presumably the open form. The free acid, obtained in solution by decomposing the silver salt under water by hydrogen sulphide, has an astringent rather than acid taste, and can not be obtained pure in the solid form, as it is not precipitated by alcohol, and the solution, on evaporating, undergoes much decomposition, leaving a gummy residue. 12 Sodium hexametaphosphimate, P6N6O12H¿Nаc + 2H2O.-The properties of this salt are essentially the same as those of the corresponding pentametaphosphimate. When containing an excess of alkali it is stable, but with a deficiency it decomposes more or less rapidly. A salt of approximately normal composition can be obtained by adding to the solution of a preparation with more than 6 atoms of sodium enough nitric acid to neutralize the excess and precipitating by alcohol (analyses 3 and 4). In the following table the results are expressed in formulas based on the determined ratio P: Na. The samples were dried at 100°. 1 and 2 precipitated from solution containing excess of alkali; 3 and 4, from solution in which excess over 5 Na had been neutralized; 5, from solution in large excess of acetic acid. Magnesium salt. - A solution of the sodium salt precipitated by magnesium nitrate in the presence of acetic acid, under exactly the same conditions as were observed in preparing the corresponding pentametaphosphimate, gave a salt which closely resembles the latter, but which does not approximate to any definite formula (found P: Mg6:2.43). The precipitation is by no means complete, much remaining in solution as a salt which can be precipitated by alcohol, which is not decomposed by boiling with alkalies, and which, in general, resembles the primary salt of pentametaphosphimic acid. Silver hexametaphosphimate, PeN6O12H6Age.-The silver salt prepared from the sodium salt varies in composition with the relative amounts of the reacting bodies and with the amount of sodium. It was obtained of normal composition by dissolving 1 gram sodium salt in 50 cubic centimeters water, adding enough nitric acid to produce a salt with 5.75 atoms of sodium, and precipitating by 50 cubic centimeters one-fifth normal silver nitrate (analyses 3 and 4). It resembles the corresponding pentametaphosphimate, but is somewhat more gelatinous. Any excess of silver over 6 atoms tends to give it a yellow color. It is decomposed in the cold by caustic potash with separation of silver oxide. The analyses given in the following table of preparations dried at 1000 show that it is derived from the true hexametaphosphimic acid, P&N6O12H12. A yellow salt is obtained by precipitating the ammoniacal solution by silver nitrate. Decomposition of hexametaphosphimic acid.-The sodium salt heated with acetic acid gives tetrametaphosphimic acid, which was isolated |