62 INTRODUCTION. 



tribes are generally few, and hence the increase of sucli 

 birds is considerably more restricted. 



Dr. Prout found the specific gravity of new laid eggs to 

 vary from 1080 to 1090 ; that eggs on being kept some time 

 became specifically lighter than water, owing to the substitu- 

 tion of air for a portion of the water which escapes ; that an 

 egg exposed for two years, to ordinary circumstances, lost 

 nearly two-thirds of its weight ; that an egg loses about one- 

 sixth of its weight during incubation; a quantity amounting 

 to eight times as much as it loses under ordinary circum- 

 stances. Although, in the size and colours of eggs of the 

 same species, there is a general conformity, yet differences 

 occasionally occur ; in some of the titmouse and tail tribe, 

 whose eggs are usually variegated with spots, they have 

 been seen perfectly white. 



There is a very simple, yet I believe not very generally 

 known, method of ascertaining the vitality of an egg. If, on 

 applying the tongue to the larger end of it, warmth be felt, 

 the egg may be presumed alive and good ; if cold, the con- 

 trary, dead and bad.* 



It should be also observed, that although the eggs of birds 

 vary considerably in taste, and some are much more palatable 

 and agreeable than others, yet none of them appears to be 

 absolutely unwholesome as food. 



In closing this short account of the incubation of birds, 

 a singular fact must be adverted to which was first brought 

 into public notice by Mr. Yaruel, a gentleman to whom 

 the public, as well as myself, are highly indebted for the 



* On my boiling inwater,forafew minutes, the egg of a Guinea 

 Hen, [Numida Meleagris,) which had been kept for ihe long 

 period of six or seven years, the egg exploded with a report 

 similar to that of a loud pistol : occasioned, no doubt, by the ex- 

 pansion of gaseous matter, arising from the decomposition of fhe 

 contents of tlie egg. 



