OS THE DEPTHS OF THE SEA. [CRA 
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 
which I have as yet examined. 
APPENDIX D. 
Note on the Carbonic Acid contained in Sea-Water. By JOHN 
Youne 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 vacwo. 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 7s 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, how- 
ever, of sulphate of magnesia and of sulphate of lime behaved like 
1 Chemical Society Journal, 1872, p. 455. 
