310 THE ATLANTIC. [chap. v. 



face specific gravity due to differences of salinity along the 

 section are very small, and that, with the exception possibly 

 of Station CCOXXIII., which is abnormal in many respects, 

 they depend mainly on the rain-fall. 



The difference between the mean surface specific gravity, 

 the temperature reduced to 15°'56 C, and the mean bottom 

 specific gravity under the same conditions is also very slight. 

 The actual specific gravity at every point is practically de- 

 termined by the temperature ; and consequently the bands of 

 equal density are, like the bands of equal temperature, virtu- 

 ally continuous with those of tlie Southern Sea. 



The Amount of Carljonic Acid contained in Sea -water. — I 

 give in Appendix C to this chapter a table of carbonic acid 

 determinations from Mr. Buchanan's " Laboratory Work ;" and 

 the substance of the few following remarks on the subject is 

 taken from his preliminary report (" Proceedings of the Royal 

 Society," vol. xxiv., p. 602 et seq.). 



The carbonic acid, when boiled out of the water, was received 

 by baryta-water of known strength ; its consequent loss of al- 

 kalinity was measured by hydrochloric acid of corresponding 

 strength. Having observed that the presence of sulphates in 

 sea-water is one of the potent agents in the retention of the 

 carbonic acid (" Proceedings of tlie Royal Society," vol. xxii., 

 p. 483 et seq.), Mr. Buchanan always added 10 cubic centimetres 

 of a saturated solution of chloride of barium to the water be- 

 fore commencing the operation. This facilitates greatly the 

 liberation of the carbonic acid, and also causes the water to boil 

 tranquilly, even to dryness, without showing any tendency to- 

 ward bumping. The quantity of water used has been almost 

 invariably 225 cubic centimetres, and the property possessed 

 by sea -water of retaining its carbonic acid with great vigor 

 makes it possible to perform the determination of it even a 

 couple of days after its collection. 



As in the great majority of cases, where the carbonic acid 



