502 Scientific Proceedings, Royal Dublin Society. 
mass. In fact it behaved very much like concrete in the manner 
of its hardening under water, though the fully hydrated salt 
MnS0,:7H.0 is soluble, as I have already stated, in so small a 
proportion as one-third of its weight of water. There are, 
however, several well-defined hydrates of this sulphate which are 
crystalline, one MnSO,2H,0, and another MnSO,5H.,0. 
Many, if not all, sulphates and carbonates which form erystal- 
line hydrates are insoluble in the anhydrous state, but become 
soluble when hydrated. This is also the case with several other 
salts; and the following are three conspicuous examples, namely, 
copper sulphate, cobalt iodide, and cupric bromide. ‘The former 
salt is white and insoluble until it has combined with water, when 
it becomes blue and dissolves. ‘The second in the anhydrous state 
is intensely black in colour; on absorbing water from the air it 
becomes the dark green dihydrate which is solid, and subsequently 
becomes a brown solution, and finally a red liquid. Conversely, 
on drying by a carefully regulated increase of temperature, the 
solution becomes a green crystallized dihydrate, and finally the 
green solid loses water, and becomes the intensely black compound, 
Col,. No solution of this black substance in water is known. 
Cupric bromide, CuBr, which is exposed to the air, absorbs 
moisture without liquefaction; the solid substance with a steely 
metallic lustre becomes the dihydrate CuBr,-2H,0, the change in 
composition being accompanied by. a great increase in volume and 
a change in appearance from a crystallized lustrous substance 
to one with a dull black appearance like charcoal, and apparently 
amorphous. The further hydration results in production of the 
pentahydrate CuBr,"5H,O in the solid crystallized form, but rise 
of temperature of but a very few degrees causes dissociation of 
the latter salt whereby the dihydrate is produced, but the water 
liberated by dissociation from the pentahydrate forms a dark 
brown solution containing either the dihydrate or, if the tempera- 
ture be raised, the anhydrous salt. It may be mentioned here that 
a hydrated cupric bromide, when dried over sulphuric acid in a 
desiccator, yielded a dull black substance with the composition of 
a monohydrate CuBr,’H.0 ; but at temperatures between 48° F. 
and 50° F. there is very little tendency to form this substance when 
the salt is exposed to moist air, the di- and tri-hydrates being 
much more readily formed. 
