80 EFFLORESCING PROPERTIES 
Hitherto I have considered those salts which 
are usually described as efflorescent salts, to be 
such as would effloresce whenever they were 
left exposed to an atmosphere which had any 
drying power; that is, to an atmosphere not 
saturated with vapour, but capable of evapo- 
rating water: and I believe that the same idea 
is generally entertained on the subject. Now, 
the result of my investigation furnishes the proof 
that such an idea is not a correct one: it shows 
that the crystallized sulphate and carbonate of 
soda, which are generally considered as very 
efflorescent salts, may be left exposed to the 
atmosphere for any length of time without 
efflorescing in the least or losing a single parti- 
cle of water of crystallization, though that 
atmosphere be dry and capable of evaporating 
water, so long as its evaporating power is not 
allowed to extend beyond a certain point :—this 
point differs for each salt. 
While my experiments were going on, I 
registered every day the temperature and the 
vapour point of the rooms in which they were 
conducted. I give a copy of the registers in a 
subsequent part of this paper. 
In order to find how much water pure anhy- 
