500 



rises to 203 C., at which point the mercury remains stationary, the 

 colourless oily acid passing over unchanged. 



Although not so violent in its action on organic matter as the pure 

 acid, the fused monohydrate, when brought into contact with wood 

 or paper, induces immediate combustion ; and when dropped into 

 water it combines with a hissing noise, forming the aqueous acid. 



3. Aqueous Perchloric Acid. 



There is certainly no other known acid in which the acid and the 

 hydrates differ so widely from each other in their properties as is 

 the case with perchloric acid, the acid itself being a strongly 

 fuming, volatile liquid, acting in a most violent manner on organic 

 substances, and closely resembling nitric acid, whilst the monohydrate 

 is a solid crystalline body, and the higher hydrates oily liquids 

 having high boiling-points, being devoid of smell, and in outward 

 appearance bearing a striking analogy to oil of vitriol. 



When pure aqueous perchloric acid is concentrated by evaporation 

 until white fumes are evolved, an acid is obtained on distillation 

 which boils at a constant temperature (about 203 C.), and has a 

 constant composition. The strength of such a residual acid was 

 determined by volumetric analysis with a standard solution of soda ; 

 it was found to contain 71 '6 per cent. C1O 4 H. As a control 

 analysis, O f 6550 grm. of the same acid was neutralized by carbonate 

 of barium, and yielded 0'5435 grm. of sulphate of barium, corre- 

 sponding to 7 1*57 per cent. II C1O 4 . Of this acid 25 cubic centims. 

 was placed in a retort and distilled until three-fourths of the liquid 

 had passed over ; analysis with soda and with barium showed that it 

 contained 72' 18 per cent. IICIO^. 



The residual acid boiling at 203 C. left in the retort after distil- 

 ling the solid monohydrate was found to possess a composition iden- 

 tical with the foregoing. Thus 1'1385 grm. of such residual acid 

 yielded 1'136 grm. of perchlorate of potassium, corresponding to 

 72-4 per rent. HC10 4 ; and 0'954 grm. of the same acid furnished 

 0'7995 grm. of sulphate of barium, or contained 72'28 per cent. 

 II C1O 4 . It is thus evident that an acid containing less H ClOj than 

 72'3 per cent, loses water, whilst an acid stronger than this loses real 

 acid on distillation ; so that both liquids yield a residual acid 

 possessing a constant composition of 72'3 per cent. II C1O 4 , and 



