The yolk contains water and albumen, but associated with these is quite a large number of mineral and other substances, which render it very complex in composition. The bright yellow color is due to a peculiar fat or oil, which is capable of reflecting the yellow rays of light, and this holds the sulphur and phosphorus which abound in the egg.
It is well known that from the egg all the constituent parts of the young animal are formed—its skeleton, as well as its various soft textures. Now, for the construction of the skeleton an amount of earthy matter is required which does not exist preformed in the soft contents of the egg, but has to be drawn from the shell. During the process of incubation, with the co-operation of the atmospheric air which permeates the shell, it appears that the phosphorus present in the yolk gradually undergoes oxidation, and becomes converted into phosphoric acid. This acts upon and dissolves the carbonate of lime belonging to the shell, which thus, as incubation proceeds, becomes thinner and thinner. The thinning of the shell also makes it easier for the young bird to peck its way out.
An enveloping membrane or bag surrounds the yolk, and keeps the fluid matter of which it is composed together. Being lighter than the white, it floats to that portion of the egg which is uppermost, but is kept in position between the two extremities by two processes of inspissated albumen, called chalazea, which pass to and are attached, one to either end of the egg.
| Entire contents. | White. | Yolk. | |
| Nitrogenous matter | 14·0 | 20·4 | 16·0 |
| Fatty matter | 10·5 | . . . . | 30·7 |
| Saline matter | 1·5 | 1·6 | 1·3 |
| Water | 74·0 | 78·0 | 52·0 |
| Total | 100·0 | 100·0 | 100·0 |
The white of egg, as this shows, contains a considerably larger proportion of water than the yolk. It contains no fatty matter, but consists mainly of albumen in a dissolved state, and inclosed within very thin-walled cells. It is this arrangement which gives to the white of egg its ropy, gelatinous state. Thoroughly shaking or beating it up with water breaks the cells and removes the ropy state.
Eggs are useful for many purposes besides food and hatching. The white of an egg has proved a most efficacious remedy for burns; seven or eight successive applications of this substance soothe the pain and effectually exclude the air from the burn. This simple remedy seems preferable to collodion, or even cotton. Extraordinary stories are told of the healing properties of an oil
