CHAP. X.| CONTINUITY OF THE CHALK. 519 
the others at first, giving off the surplus carbonic acid dissolved 
in the first eighth of the distillate. The amount of carbonic acid 
coming off then fell very low, gradually increasing, however, 
until a half had been distilled over, when the amount coming 
off again reached a maximum, the quantity then diminishing, 
but rarely entirely disappearing as the contents of the retort 
approached dryness. It is clear, then, that in the sulphates of 
magnesia and lime we have an agent capable of retaining car- 
bonic acid in the way in which we see it in sea-water; whether 
there may be other agents present, capable of doing the same 
work, will be brought to light when the subject has been more 
fully investigated. An independent set of experiments were 
made on the variation with pressure of the coefficient of ab- 
sorption for carbonic acid of a solution containing 1°23 per cent. 
of crystallized sulphate of magnesia, kept at a constant tem- 
perature of 11°C. The result was, that at 610 mm. pressure the 
sulphate of magnesia solution dissolved sensibly the same quan- 
tity of carbonic acid as the same volume of water would have 
done ; in other words, their coefficients of absorption were iden- 
tical. Below 610 mm. that of the saline solution was the greater ; 
above 610 mm. the reverse was the case. The curve, however, 
is not a straight line, and it appears to cut that of water again 
at a pressure of about 800 mm. 
The facts above related naturally suggest to the chemist the 
question, what is the body formed when sulphate of magnesia 
and carbonic acid meet each other in solution ? 
It is clear that, besides the carbonic acid dissolved, there is 
some retained by a stronger bond, and which is only liberated 
when the concentration has proceeded a certain distance. Is 
the decomposition caused by the loss of water, or by the rise of 
boiling-point ? The difference between the boiling-points of the 
solution, when it has just ceased to give off the merely dissolved 
carbonic acid, and when the retained gas is being given off in 
greatest quantity, does not exceed 1° C.; and it is difficult to 
believe that the compound should remain practically intact at 
101° and decompose rapidly at 102°. Again, if the compound 
is decomposed by the water alone, we should expect, that the 
