OF SOME SALTS OF SODA. 97 
under the receiver, they do not regain their 
transparency though they all regain their hard- 
ness, with the exception of the pyro-phosphate 
which only does so to a slight amount. If, how-. 
ever, dew happens to fall upon them they do 
regain their transparency, and to an amount 
proportionate to the quantity of dew which falls 
upon them. If an exhausted receiver having a 
vessel of water under it suffers a reduction of 
temperature, by being in a room whose temper- 
ature is allowed to fall, a precipitation of dew 
takes place within it; and this falling upon the 
salt, the object of experiment, not only has a 
tendency to bring back its transparency, if it 
has already got its full quantity of water of crys- 
tallization, but renders it damp :—if this is 
allowed to take place in the instance of pyro- 
phosphate of soda that salt does not even then 
take into combination any more water than I 
have described ; for, if to dry it, it be exposed 
to an atmosphere whose drying power is not 
sufficiently great to cause efflorescence all the 
excess of water evaporates. 
I apprehend that the circumstance of anhy- 
drous sulphate of soda gaining water till it be- 
comes of the same constitution as the crystal- 
lized salt may be taken advantage of in analysis. 
F 
