One cubic centimeter of the acid prepared in this way, on evaporation gave a residue in one case of 0.0369 gram, and in another 0.0307 gram, completely soluble in ninety-seven per cent alcohol, which was then ignited and the chlorin determined by silver from which the equivalent of perchloric acid in the form of salts was calculated as 0.0305 gram. By neutralizing the acid with sodium carbonate, evaporating, igniting in an atmosphere of carbon dioxid till decomposition was complete, collecting the oxygen over caustic potash, allowing it to act on hydriodic acid by intervention of nitric oxid, according to a process soon to be published, titrating the iodin liberated, with standard arsenic and calculating the equivalent of perchloric acid, after subtracting the amount of acid found in the form of salts, the amount of free acid per cubic centimeter proved to be 0.9831 gram. The whole process, even when the separation with alcohol is necessary, can not well require more than two days and during the greater part of that time the work proceeds without attention. =273. Keeping Properties of Perchloric Acid.=—By most authorities it is asserted that perchloric acid is a very unstable body and is liable to decompose with explosive violence even when kept in the dark. It is probable that this tendency to spontaneous decomposition has been exaggerated. It is not even mentioned in Gmelin’s Handbook.[228] The most concentrated aqueous acid has a specific gravity of 1.65, is colorless, fumes slightly when exposed to the air, and boils at 200°. It has no odor, possesses an oily consistence and has a strong and agreeably acid taste. It reddens litmus without bleaching it and is slowly volatilized at 138° without decomposition. It is unaffected by exposure to the light, even the sun’s rays. It is not decomposed by hydrosulfuric, sulfurous, or hydrochloric acids, nor by alcohol. Paper saturated with the strong acid does not take fire spontaneously, but it deflagrates with red-hot charcoal. The acid prepared by the method of Kreider has approximately the composition of the di-hydrate, HClO₄·2H₂O.[229] Unless well evaporated, however, it is a little more dilute than is shown by the above formula. The di-hydrate is quite stable and the more dilute acid can be kept for an indefinite time. Kreider has kept the acid for six months and noticed no change whatever in its composition. Acid containing one gram of perchloric acid in a cubic centimeter has been kept three months with perfect safety. There is no reason why the strong aqueous acid should not be made a regular article of commerce by dealers in chemical supplies, under proper restrictions for storage and transportation. The strong acid made in this laboratory by the Kreider method has not given the least indication of easy or spontaneous decomposition.