362 RELATIVE ATTRACTIONS OF 
I have stated, that on the 6th and 7th the con- 
centration of the acid had extended so far as to 
cause the per centage of real, or anhydrous, acid 
in the residue in the dish to be about 26.78 ; acid . 
of such strength is of about the sp. gr. 1.249. 
Hence, it follows that when the affinity of space 
for vapour, or the evaporating force, is equal to 
0.15 of an inch of mercury, it is just able to 
balance the affinity of sulphuric acid sp. gr. 1.249 
(or such at least as contains 26.78 per cent. of 
real) for water. 
It is well known that ordinary evaporation 
from water goes on at a greater rate in vacuo 
than in air under atmospheric pressure ; and it 
might obviously be expected that the rate at 
which water would evaporate from diluted sul- 
phuric acid would be greater in vacuo than in air; 
but, at the conclusion of the experiments I have 
related, it seemed to be matter worth investiga- 
tion to ascertain whether the evaporation of water 
from diluted sulphuric acid was capable of being 
carried on to the same extent in air as in vacuo; 
or, in other words, whether the same strength of 
acid was required to render air at its usual pres- 
