Carbonic Acid by Saline Solutions. 489 



Under the most favourable circumstances, then, one would expect sea- 

 water to bind about one third of the quantity retained by an equal 

 volume of sulphate-of-lime solution. We have seen that a litre of this 

 solution is capable of retaining 0-338 grin. C0 2 , while the same volume 

 of sea-water contained only O07 grm., or considerably less than the third 

 of that held by the sulphate of lime. In ocean-water I have never yet 

 found more than O064 grm. CO 2 per litre, including both the simply 

 dissolved and the 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. Besides this there is also, at least, the sulphate 

 of magnesia possessing this property. How much it would be capable of 

 absorbing if the carbonic acid were presented in a nascent state in a 

 neutral solution we do not know ; it would be interesting to determine 

 the amount of carbonic acid retained by a sulphate-of -magnesia solution 

 in which organic matter had been allowed to decay. 



The practical conclusion to be drawn from the preceding experiments 

 is that, as the carbonic acid is retained by the presence of certain sul- 

 phates, the gas will be more easily boiled out if we get quit of these 

 sulphates. For this purpose I always add to the sample of sea-water, 

 in which the C0 2 is to be determined, a sufficient quantity of a saturated 

 chloride-of-barium solution to precipitate all the sulphuric acid present. 

 The effect has answered my expectations. After the first fifth of dis- 

 tillate has passed, there is rarely a perceptible turbidity in fresh baryta- 

 water. In practice, however, and as it costs but little trouble, I always 

 distil off from three quarters to seven eighths, and often quite nine 

 tenths of the solution. 



The determination of the carbonic acid in sea-water is carried on on 

 board the ' Challenger ' by means of an apparatus, a very slightly mo- 

 dified form of the one described by Dr. Jacobsen in the ' Annalen der 

 Chemie und Pharmacie,' a drawing and description of which he was good 

 enough to give me when the ' Challenger ' was fitting out. 



A flask with a capacity of about 500 c. c. receives the sea-water to be 

 operated on, usually from 200 c. c. to 250 c. c. It is closed by an 

 india-rubber cork, through which pass two tubes ; one, reaching to the 

 bottom, communicates with the condenser, a cylindrical copper vessel, 

 10 in. high by 5-J in. diameter, with a block-tin worm. The lower end 

 of the worm is attached to the receiver by a bent glass tube with a 

 flexible joint, from which a glass tube leads to the bottom of the 

 receiver. The flexibility thus obtained is, in practice, of the greatest 

 use, enabling the operator, by shaking, to expose constantly fresh sur- 

 faces of baryta- water to the passing gases. The receiver is connected by 

 an india-rubber tube with two bulbed V-tubes. An aspirator enables a 

 stream of air to be drawn through the apparatus, a soda-lime safety-tube 

 being interposed between it and the V-tubes. The water running from 

 the aspirator is conducted outside the port by a tube which passes 



