386 RELATIVE ATTRACTIONS OF 
sp. gr. 1.6405, the rate of evaporation from acid 
sp. gr. 1.45 or 1.50 at the same temperature, must 
be but very slow, and more especially so at tem- 
peratures below 36°. Sometimes in severe winter 
weather the temperature of the chambers is pro- 
bably as low as 17°; and my experiments show 
that when the temperature is only 17°, and the 
pressure about 29.8 inches of mercury, the acid 
can only be concentrated so far as to have the 
per centage 26.78, =sp. gr. about 1.249 ; the fact, 
therefore, is, that in a case of so low a tempera- 
ture as 17°, no evaporation whatever could take 
place from the acid, if its sp. gr. was greater than 
1.249; and, consequently, in such a case, the 
process of the manufacture of sulphuric acid must 
be entirely stopped, was it not that aqueous vapour 
was supplied from some other source. Indeed, 
some vapour always is supplied otherwise than 
from the acid on the floors: some enters the 
chambers with the air by which the combustion is 
supported; but, when the temperature of that air 
is so low as 17°, or 20°, or 30°, the weight of the 
vapour admitted along with it is too trifling to be 
of much avail in an instance of so great a demand. 
It has for some time been a practice among some 
manufacturers of the acid, to turn steam, issuing 
from a pipe connected with a vessel of boiling 
