518 THE DEPTHS OF THE SEA. [chap. x. 



probably is a mass of hornblendic gneiss or schist, and certainly 

 not of true volcanic origin. I may mention that it does not at all 

 resemble any of the fragments found in the deep-sea dredgings 

 whicli I have as yet examined. 



APPENDIX D. 



Note on the Carhonic Acid contained in Sea-water. By John 

 Young Buchanan, M.A., Chemist to the ' Challenger ' 

 Expedition. 



At a meeting ^ of the Chemical Society last summer, Dr. 

 Himly mentioned that Dr. Jacobsen, of Kiel, had found that 

 carbonic acid is only very imperfectly separated from sea-water 

 by boiling in vacuo. This was confirmed by Dr. Jacobsen him- 

 self in a letter to Nature of August 8, 1872. Almost at the 

 very same time the German North. Sea Expedition arrived in 

 Leith, when I had the privilege of hearing the confirmation of 

 it from his own mouth, as well as his conjecture that it was 

 probably owing to the presence of salts with water of hal- 

 hydration, such as sulphate of magnesia, that the carbonic acid 

 was retained with such vigour. 



Having assured myself by experiment that, as a matter of 

 fact, carbonic acid is retained by sea-water with considerable 

 energy, the last traces of it having scarcely disappeared before 

 the contents of the retort were reduced to dryness, I set on foot 

 a series of analytical experiments, so as to determine which of 

 the salts it was, whose presence was the cause of the anomaly in 

 question. The result of these experiments was shortly this: 

 Distilled water, solution of chloride of sodium and solution 

 of chloride of magnesium, each saturated with carbonic acid, 

 behaved on distillation alike, giving off the whole of their car- 

 bonic acid in the first eighth, of the distillate. Solutions, however, 

 of sulphate of magnesia and of sulphate of lime behaved like 



' Chemical Society Journal, 1872, p. 455. 



