92 EFFLORESCING PROPERTIES 
The vapour point was ascertained by the aid 
of an hygrometer on the principle of Leslie’s, 
(an abstract of a paper of mine respecting which 
will be found in the report of the meeting of 
the British Association at Edinburgh,) though 
I frequently found it experimentally, to prove 
the correctness of the instrument. 
Supposing that the increase of atmospheric 
temperature increases the disposition of the 
water of crystallization to evaporate from salts 
according to the same law as it increases the 
disposition of ordinary uncombined water to 
evaporate,* we shall be able to determine from 
the result of my experiments and by the aid of 
Dr. Dalton’s table of the force of vapour from 
water at different temperatures, at how low a 
vapour point these salts may be exposed to the 
atmosphere at other temperatures than what 
my experiments were made at, without suffering 
a loss of water. 
I have stated that I find the crystals of carbo- 
nate of soda only begin to effloresce at the tem- 
perature of 58° when the vapour point is at 48°: 
* The capacity of the atmosphere for vapour increases with 
the increase of temperature, and decreases with the decrease 
of temperature. 
