246 DR DAVY ON THE SALMONIDiE. 



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 half a 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. Biot, 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 Salmonidse, 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 



