PREPARATION OF PERCHLORIC ACID. 
313 
adopted of rejecting the portion first distilled over, which is 
nothing but pure water. 
Notwithstanding the very diluted state of the perchloric 
acid obtained in this way, still it reacts very well on the salts 
of potash; it is this probably that has led to the belief, that 
the whole of the salt contained in the retort had been decom- 
posed, and that the perchloric acid obtained, represented 
very nearly the salt employed, which is far from being the 
case, independent of the partial decomposition of the per- 
chlorate into its elements by the mere action of the heat. 
If the operation be well observed, it will be seen that the 
water added to the sulphuric acid is Useless, or nearly so, and 
that the quantity of sulphuric acid is insufficient for the de- 
composition of the whole of the perchlorate; because here, 
contrary to the ordinary law of substitutions, the influence of 
mass, a sort of union of forces, if I may be allowed the ex- 
pression, is required to overcome the pre-existing affinity; the 
perchloric acid being capable, under certain circumstances, to 
displace the sulphuric. Having arrived at this point, it only 
remained to determine the proportion of sulphuric acid neces- 
sary to decompose the whole of the perchlorate. This pro- 
portion I will now give, and although at first sight, it may 
appear arbitrary, it is nevertheless indispensable for obtaining 
the whole of the perchloric acid. 
Introduce into a glass retort 500 parts of perchlorate of 
potash, deprived as much as possible of any chlorate, and re- 
duced to powder; add 1000 parts of sulphuric acid at 66°, 
free from nitric acid; then add 100 parts only of distilled 
water. This small quantity of water is not indispensable, for 
it will be seen further on, that by omitting it, the perchloric 
acid is immediately obtained in the solid form, in crystals. 
Fit to the retort a long adapter, terminating in a tubulated re- 
ceiver, surrounded with cold Water. It is necessary to avoid 
luting the apparatus with paper, or any other organic sub- 
stance, for this, when heated and in contact with the vapors 
of perchloric acid, would soon cause a decomposition, accom- 
panied with slight detonations: if the apparatus be well ad- 
