OF SOME SALTS OF SODA. 87 
sequence of the lowness of the vapour point on 
the 2nd (11° below the temperature), and also 
on the 4th (10° below the temperature): on the 
ilth, these crystals were no more effloresced 
than they were on the 5th: by the 4th Decem- 
ber, however, efflorescence was considerably 
more apparent, though even then it had not 
made a great advance: in the interval between 
the 11th November and the 4th December, the 
vapour point had several times been 10° and 11° 
below the temperature; which was undoubt- 
edly the cause of this further efflorescence. 
Though this efflorescence, or rather opacity 
of the outside crystals had taken place, the weight 
was still the same as when they were transparent, 
viz. full 20 grains. This is accounted for by 
supposing that the crystals lose water only when 
the vapour point is a certain distance below the 
temperature, and that they gain back that which 
they then lose when the vapour point is a less 
distance below the temperature: or, when the 
atmosphere is of a certain drying power it robs 
the crystals of water; and when it is of a less 
drying power it allows them to regain their loss. 
From the 4th November to the 4th December, 
{ had also 20 grains of anhydrous Carbonate of 
