INCUBATION. 221 



zoic acid, and sulphureted hydrogen gas. (Bostock's Phys- 

 iology.) 



The latter, we observe, on eating an egg with a silver spoon, 

 to stain it of a blackish purple, by combining with the silver, 

 and forming sulphuret of silver. 



The yolk has an insipid, bland, oily taste ; and, when agi- 

 tated with water, forms a milky emulsion. If it be long boiled, 

 it becomes a granular, friable solid, yielding, upon expression, 

 a yellow, insipid, fixed oil. It consists, chemically, of water, 

 oil, albumen, and gelatine. In proportion to the quantity of 

 albumen, the egg boils hard. 



The white of the egg is found to be a very feeble condu-Aor 

 of heat, retarding its escape, and preventing its entrance to the 

 yolk ; a contrivance of Providential Wisdom, not only to pre- 

 vent speedy fermentation and corruption, but, as Dr. Paris 

 remarks, to avert the fatal chills which might occur in hatch- 

 ing, when the mother hen leaves her eggs, from time to time, 

 in search of food. Eels, tench, and other fish, which can live 

 long out of water, secrete a similar viscid substance on the 

 surface of their bodies, furnished to them, no doubt, for a sim- 

 ilar purpose. (Linn&an Transactions, x., 306.)" Dickson. 



The process of incubation, now to be described, is exceed- 

 ingly curious, and has attracted great attention from natural- 

 ists. By the use of artificial methods of hatching, all the 

 changes which the egg undergoes have been discovered, and 

 the results of observation have been recorded with great pre- 

 cision. The usual time of incubation is twenty-one days, 

 though sometimes the chick is excluded on the eighteenth 

 day. 



The changes which the egg passes through in hatching may 

 be briefly described from the statements of Dickson. In twelve 

 hours, traces of the head and body of the chicken may be 

 observed as in the cut. 

 19* 



