14 ORIGIN OF ROCK-SALY. 
the bromine and potash salts, which in the German salt mines 
occur so freely as to be worked for commercial purposes. In fact, 
every item in sea-water down to iodine, has been found, when 
sought for, in the rock-salt beds. This is the first chain in the 
link of evidence. 
It might be supposed that if rock-salt is due to the drying up 
of salt-water, we ought to find the several ingredients, not as 
we do in consecutive layers, but mixed together in a confused 
mass. Such might seem to be a proper inference to anyone not 
acquainted with the laws which govern the deposition of those 
ingredients. 
What actually occurs when the salts of sea-water are separated 
by evaporation may be known by experiment, and is as follows : 
We may be supposed to be operating upon a considerable body 
of sea-water. The first application of heat drives off the gascous 
matters, including the carbonic acid gas. When 46 per cent. of 
the water has been evaporated, by the disappearance of the 
carbonic acid, the iron is reduced to oxide and quickly falls to 
the bottom, and if clay be present, unites with it, giving to it 
the reddish tint common to the marl. Continuing the boiling 
process, when 70 per cent. of the water has disappeared, the ~ 
bicarbonate of lime parts with an equivalent of carbonic acid, 
and becoming the insoluble carbonate of lime, falls as a powder 
to the bottom. When 90 per cont. of the water has gone, the 
sulphate of lime having reached its point of saturation, falls to 
the bottom also. When the water has lost 6 per cent. more, it 
comes to the turn of the chloride of sodium, and as the largest 
ingredient present, it forms a thick layer on tho sulphate of lime. 
Lastly, at 98 per cent., the salts of magnesia with the bromine 
are thrown down. No one can fail to admire the regular order 
in which the various salts have left the sea-water: each has 
been held in its place by its law of solution. When the water 
can no longer hold the salt it falls to the bottom. This experi- 
ment will explain how it is that when salt-water deposits its 
saline particles, it does not lay them down in a promiscuous, but 
in a consecutive manner, 
