497 



Perchloric acid is a heavy, colourless, volatile liquid, which does 

 not solidify at temperatures above 35 C. At 15'5 C. the specific 

 gravity of the liquid was found to be T782 as a mean of two deter- 

 minations. The vapour of perchloric acid is colourless and trans- 

 parent ; but when brought into contact with the air, dense masses of 

 white fumes of hydrated acid are given off, owing to rapid absorption 

 of atmospheric moisture. Perchloric acid does not give up its oxygen 

 to metals at the ordinary temperature as readily as nitric acid ; but 

 when heated with finely divided iron, copper, zinc, &c., the oxides of 

 these metals are formed, with evolution of light and heat ; sulphur and 

 phosphorus are likewise at once converted into their highest oxides 

 in presence of this acid. It is, however, when brought into contact 

 with a carbonized material that the energy of the oxidizing action of 

 perchloric acid becomes visible, as a single drop of this substance 

 falling upon charcoal, paper, or wood, causes an explosive combustion 

 which in violence does not fall far short of the detonation produced 

 by the sudden decomposition of the chloride of nitrogen. Dropped 

 into ether or alcohol, an explosion occurs violent enough to break in 

 pieces the vessel containing the liquid. This latter decomposition 

 may possibly be owing to the formation of the explosive body, 

 described by Messrs. Hare and Boyle as the perchlorate of ethyl. 

 Perchloric acid likewise combines with the greatest energy with water, 

 causing a loud hissing noise, and forming either the crystallized or 

 the oily acid, according to the relative quantities of water and acid 

 present. In working with this substance the greatest care must be 

 taken, as one drop falling upon the skin produces powerful cauteriza- 

 tion, and leaves a wound which does not heal for weeks. Like 

 nitric acid, NO 3 H, which in many respects it closely'resembles, per- 

 chloric acid cannot be distilled by itself under the ordinary atmospheric 

 pressure without undergoing decomposition. The colourless liquid, 

 when gently heated, begins to decompose at 75 C., the temperature 

 gradually rising to 92, at which point it remains constant for some 

 time, gives off dense white fumes, and quickly turns to a dark blackish- 

 brown colour, large quantities of a yellow gas having the peculiar 

 sweetish smell of chloric oxide being evolved. After some time drops 

 of a dark-coloured liquid, resembling bromine in appearance, distil 

 over, the decomposition continuing in the retort ; this dark liquid 

 contained 94' 77 per cent, of C1O 4 H. On continuing the distillation 



