282 FACULTIES OF BIRDS. 
ter, but remain in the island, and issue from their 
retreat in warm days in quest of food.* 
As the dormouse, the bat, and other hybernating 
animals do not appear to have any peculiarity of 
anatomical structure from which we might account 
for the circumstance, besides certain depositions of 
fat and a valvular conformation of the veins, the 
use of which is not well known, we cannot, conse- 
quently, make any inference upon this point from 
the anatomical structure of swallows and other mi- 
gratory birds. But experiment is no less valuable 
a test than structure, and experiments respecting 
the conjectured torpidity of the swallow were tried 
by Spallanzani, who found that swallows do not ap- 
pear to suffer by cold at the freezing point; while 
at eight or nine degrees below it they manifest un- 
easiness, and at thirteen or fourteen degrees below 
it they speedily perish. In order to discover the 
effect of a continued low temperature, Spallanzani 
confined some swallows in wicker cases covered 
with waxed silk to keep them dry, burying them in 
snow, with only a hole to admit air. * After having 
been immersed for thirty-five hours, some of them 
were dead, and others exhibited signs of great 
weakness, but without any appearance of torpidity 
or even lethargy ; in ten hours more they were all 
found dead. That they had not died in consequence 
of want of food, he further proved by keeping other 
swallows without food in his study, when he found 
they could support life from three to five days with- 
out anything to eat. % 
A still more convincing proof that swallows do 
not become torpid in winter, may be derived from 
those which have been successfully kept in cages. 
Dr. Reeve says he has known several attempts 
made to keep swallows in a warm room during 
winter without success; but M. Natterer kept a 
number of swallows in cages for eight or nine 
* Travels in the Two Sicilies, iv., 115. 
