246 DR DAVY ON THE SALMONIDZ. 
tation with lime-water with excess of lime. The proportion of this acid gas was 
in every instance small, barely a trace. For oxygen the test used was a stick of 
phosphorus, left exposed to the action of the air some time after it had ceased 
to fume, or the air to suffer diminution. In the instance of the salmon mentioned, 
the diminution from the action of the phosphorus was hardly appreciable. The 
same remark applies to the air of the bladder of the white trout: it was tried in 
two instances,—fish of about halfa pound caught in the Claudy river in Donegal. 
In the trout, river-trout, fish of about a quarter of a pound (two were examined), 
the proportion of oxygen was greater; it amounted to about ten per cent. of the 
whole volume of air. 
As these trials were mostly conducted when on fishing excursions, and under 
circumstances nowise favourable for minuteness of research, the experiments I have 
made on the air were chiefly limited to those above described, which sufficed to 
convince me that the air of the air-bladder was principally azote, and to allow the 
inference that the trace of carbonic acid was most likely rather accidental than 
essential, owing probably to the secondary action of the minute proportion of 
oxygen present on the organ itself. 
These results, I may remark, accord with those obtained by former inquirers 
on the air of the air-bladder of several other fresh-water fish, whilst they differ 
so greatly from others—those afforded in like trials on deep-sea fish,—the air of 
the air-bladder of which was found to be principally oxygen. 
That the same organ should secrete two gases so very different in their na- 
ture, appears anomalous, and deserving of further inquiry. Indeed, does not the 
entire subject need more minute inquiry? At present, the facts relating to it are 
few, and seem far from adequate to allow of any satisfactory conclusions being 
drawn as to the use of the bladder and its secretion in the animal economy, ex- 
cept of a mechanical kind, as affecting the specific gravity of the fish. Were the 
gas uniformly of one kind, were it constantly azote, it might be easy to assign it 
a plausible end; the function of the air-bladder might be inferred to be auxiliary 
to that of the kidneys. The secretion of oxygen is the anomalous fact, so contrary 
is it to the ordinary course of changes in living animals, in which the general 
tendency is to the consumption of oxygen. A priori, one might almost as much 
expect oxygen to be exhaled from the lungs in respiration, as to be separated from 
the blood by secretion by the air-bladder; and, had we not the authority of so 
accurate an observer as M. Brot, we might be led to suspect that the statement 
of its being so was founded in error. 
2. Of the Abdominal Aperture of the Female. 
In a paper on the impregnation of the ova of the Salmonidee, which I had the 
honour of presenting to the Society last year, I expressed the opinion that the 
passage through which the ova have their exit is not constantly open; that after 
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