CHAP. x.J 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 t 

 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 T23 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 



