1874.] Carbonic Acid by Saline Solutions. 195 



From the large amount of organic matter poured into the Forth in the 

 neighbourhood of Portobello, there must be an abundant production of 

 carbonic acid in the water itself ; and we have seen above the effect of 

 bringing it together in the nascent state with sulphate of lime. Sea- 

 water contains on an average about 8 parts sulphate of lime in 10,000. 

 A saturated solution of the saine salt in distilled water contains, at 

 15 0., 24 parts in 10,000. Under the most favourable circumstances, 

 then, sea- water might be expected to bind about one third of the quantity 

 retained by an equal volume of saturated gypsum solution. We have 

 seen that a litre of this solution is capable of retaining 0'338 grm. CO 2 , 

 while the same volume of sea-water contained at the most only 0'07 

 grm., or very little more than one fifth of that held by the sulphate of lime. 



In ocean-water I have never yet found more than 0'064 grm. CO 2 

 per litre, including both simply dissolved and half bound. We have, then, 

 in the sulphate of lime alone an agent capable of retaining much more 

 carbonic acid than is usually found to exist in sea-water ; and there is 

 besides the sulphate of magnesia ; so that whatever may be the function 

 of the other salts, we do not, in order to find a vera causa for the phe- 

 nomenon under consideration, require to go beyond the sulphates ; and 

 the practical lesson to be learned is that, if we get quit of the sulphates, 

 the carbonic acid will be more easily disengaged by heat. 



This is entirely borne out by experiment. In determining the car- 

 bonic acid in sea-water, I always add to it a sufficient quantity of a satu- 

 rated solution of chloride of barium ; and I find that, after about the 

 first fifth of the distillate has passed, there is rarely a perceptible tur- 

 bidity in fresh baryta-water. 



The synthetical experiment consisted in determining directly the 

 coefficients of absorption of a 1*23 per cent, solution of crystallized 

 sulphate of magnesia and of a 0'205 per cent, solution of Ca SO 4 

 + 2H 2 O. In Table I. the results of experiments on the magnesia so- 

 lution are given, where the observations were made without loss of time. 

 In Table II. the results of experiments on the same solution are 

 given, only here the duration of the reaction was taken into account. 

 The first reading was made at the highest pressure after the gas and 

 solution had been together for nine days ; the pressure was then succes- 

 sively reduced, and the other readings made at intervals of twenty-two, 

 forty-one, and twenty-five hours from each other, the last of all being 

 made only after the lapse of some days. Table III. gives the results of 

 experiments on the gypsum solution, the readings in this case being 

 made without allowing much time for the reaction to take place. 



TABLE I. 



