166 ESSAYS anp OBSERVATIONS 
is to be imputed rather to their mixing 
with the water of the air, than to their e- 
vaporation fingly. This perhaps cannot 
be pofitively determined, till the evapora- 
tion of thefe acids, in a very perfect va- 
cuum, is examined ; which I have not yet 
had an opportunity of doing. In the mean 
time, I have made an experiment which 
I think is to the purpofe. To one part of 
ftrong acid of vitriol, I added two parts 
of {pring-water. When this mixture, 
which produces a great degree of heat, 
was returned to the temperature of the air, 
I ufed it for moiftening the ball of the 
thermometer, and found it produced a fen- 
fible degree of cold, and feemingly a great- 
er than water alone would have done. I 
need not here obferve, that the mixture I 
ufed was ftill a very acid liquor, only fo 
much faturated with water, that it would 
not now attract any from the air. Whe- 
ther it would not have had the fame effed, 
tho’ lefs diluted, I have not had time to 
examine. ‘The experiment, as it ftands, 
tends to prove, that the heat produced by 
acids, applied to the ball of the thermo- 
meter, 
