254 
abundance of food, other circumstances aiding. The relation of the 
size of the egg of different species to that of the parent birds is 
suggested as an interesting subject for inquiry ; and the conjecture is 
offered, illustrated by some examples, that the circumstances chiefly 
influencing that relation may be the manner in which the young 
birds are fed, whether by the old birds, or unaided by them, and 
the proportional abundance of food. 
On the Specific Gravity of Birds. — The birds submitted to trial 
were the martin, the water-ouzel, snipe, wood-owl, merlin-hawk, 
and wren. These, deprived of their feathers, were found to be of 
nearly the same specific gravity as the water in which they were 
weighed. With the feathers on, in common with birds generally, 
they all floated in water, buoyed up by the lightness of their feathers, 
- — these so light owing to the air they contain. As the air becomes 
disengaged on forced submersion, so the specific gravity of the bird 
increases. A wren, the specific gravity of which w'hen first sunk 
in water was 0*890, after a continuance under water of twelve 
hours, had increased to 0 960. The specific gravity of the water- 
ouzel with its feathers on was found to be about 0*724 ; that of the 
merlin-hawk about 0*570. It is pointed out, in conclusion, how r 
little the specific gravity of the bird is concerned as regards apti- 
tude for aerial locomotion, and how it is subordinate to various cir- 
cumstances, such as lightness from included air and a high tempera- 
ture, and a great impulsive power of wing. 
On the Stomach of Fish in relation to Digestion . — In the in- 
stances of the salmon and sea-trout, and indeed of all the fish tried — 
these including the common trout, charr, grayling, haddock, and 
dogfish — the stomach, when empty, was found by test-papers to 
be commonly neutral, and on the contrary acid when any food was 
present; leading to the conclusion that the gastric juice is secreted 
only when the organ is stimulated and that fluid is required. 
From observations on fish after death, kept for a certain time, suffi- 
ciently long for the fluids to act on the containing solids, denoted by 
softening and rupture, the inference is come to that the fluid of the 
appendices pyloricse is most active in producing the post-mortem 
effects ; and that the migratory species of the salmonidse, the salmon 
and sea-trout, taking little food in fresh water, exhibit least this 
effect, and are least subject, from the emptiness of their primee vise, 
to putrefaction. 
