CHEMISTRY OF A HEN'S EGG. 23 



materials of the manufacture are found in the food 

 consumed, and in the sand, pebble-stones, brick- 

 dust, bits of bones, etc., which hens and other birds 

 are continually picking up from the earth. The 

 instinct is keen for these apparently innutritious 

 and refractory substances, and they are devoured 

 with as eager a relish as the cereal grains or in- 

 sects. If hens are confined to barns or out-build- 

 ings, it is obvious that the egg-producing machinery 

 cannot be kept long in action, unless the materials 

 for the shell are supplied in ample abundance. 



Within the shell the animal portion of the egg 

 is found, which consists of a viscous, colorless liquid 

 called albumen, or the white, and a yellow, globular 

 mass called the vitellus, or yolk. The white of the 

 egg consists of two parts, each of which is en- 

 veloped in distinct membranes. The outer bag of 

 albumen, next the shell, is quite a thin, watery 

 body, while the next, which invests the yolk, is 

 heavy and thick. But few housekeepers who break 

 eggs ever distinguish between the two whites, or 

 know of their existence even. Each has its. appro- 

 priate office to fulfil during the progress of incuba- 

 tion or hatching, and one acts, in the mysterious 

 process, as important a part as the other. If we 

 remove this glairy fluid from the shell and place it 

 in a glass, and plunge into it a strip of reddened 

 litmus paper, a blue tinge is immediately produced, 



