Yolk And White Of Egg

We will first consider the composition of the yolk and white of ordinary hen's eggs. The percentage proportions are, on the average:

The white, it will be seen, is characterized by the presence of 12 parts per hundred of albumen, which is in solution in the ropy liquid. When this solution is heated to a temperature considerably below that of boiling water, the albumen becomes insoluble, and is said to be coagulated; it is not capable of being again dissolved in its original menstruum. Solutions of tannin, corrosive sublimate, and many other compounds, inorganic and organic, produce a similar effect. But egg-white is not a pure solution of albumen. For all practical purposes in the arts, it may be sufficiently freed from extraneous matters in the following manner: The necessary number of 'whites' are mixed in a wide-mouth stoppered bottle, with twice their bulk of water, and shaken up thoroughly; then a slip of yellow turmeric-paper is dropped into the mixture. Drop by drop weak acetic acid is poured in, until the reddened turmeric-paper has just, or nearly, regained its original yellow hue. In this way the alkaline reaction of the liquid is almost neutralized, and it becomes thinner. After further agitation, the mixture is poured upon a piece of well-washed muslin in a funnel. The clear liquid which drops through has been freed from membranes, etc., and contains nearly 4 percent of albumen.

It may be concentrated by cautious evaporation at a temperature not exceeding 50° C. The albumen which it contains is a very complex substance, containing, besides carbon, hydrogen, nitrogen, and oxygen, about 1.6 percent of sulphur. A solution of albumen spread upon glass, and allowed to dry slowly at the ordinary temperature, leaves a residue of albumen in the form of a nearly-transparent film. This, when quite dry, is brittle, and easily cracks. If, before it be quite dry, it be heated to 70°or 75° C., it cannot be again dissolved by water, having been converted into the insoluble form. In this condition it is much less prone to change. It will now be seen how powdered pigments, if ground up with albumen solution and then used in painting, may be made to cohere, and also to adhere to the painting-ground of cloth, paper, or plaster, on which they have been spread. And afterwards, by simply heating the work sufficiently, the whole coloured layer may be rendered insoluble and irremovable by water. Advantage may also be taken of the action of tannin on albumen to secure the same result - the coagulation of the albumen.

We may coat a piece of fine linen cloth with albumen-solution, and before it is quite dry we may paint upon it with pigments which have been previously ground up with a weak solution of tannin. If the work be carefully done, the colours will, when dry, be found to have been fixed by the reaction between the tannin and the albumen. If, however, the pigments be laid on somewhat quickly, it may be found necessary to give the whole surface a final coat of albumen-solution. We have dwelt at some length upon this employment of tannin, or of heat, to secure the coagulation of albumen, because it serves to illustrate the way in which paintings, executed with egg-yolk, or size, as a medium, may be fixed. For, as we shall now proceed to show, egg-yolk and size possess many characters in common with albumen-solution.

But the yolk of an egg contains other substances besides albumen. First of all, the albumen present is accompanied by another similar compound called vitellin, which resembles it in composition and properties, and which, for our present purpose, we need not further describe, except so far as to state that, unlike albumen, it is not soluble in water. Of albumen and vitellin, taken together, egg-yolk contains, as we have seen, not less than 14 or 15 percent But egg-yolk is something more than a solution of these two similar bodies. It is, in fact, an oily emulsion, in which innumerable minute globules of a thick, fatty oil are suspended in an albuminous solution. And, moreover, the amount of this oil is large; there is about 22 percent of it, and associated with this oil there is no less than 9 percent of a curious compound called lecithin, which has many of the physical properties of a fat. It seems to be a triglyceride, including two fatty-acid radicles and one phosphoric acid radicle. Associated with lecithin there is a nitrogenous basic compound. Although lecithin resembles oils and fats in its behaviour to most solvents, it yet differs from them in this one particular, that it is very hygroscopic and swells up in water, forming a kind of emulsion.

Now, 9 parts of lecithin with 22 parts of oil make up nearly one-third of egg-yolk, or 31 parts of oily or fatty matter per 100, as against 15 parts of albuminoid matter, or vitellin and albumen taken together. Hence it happens that egg-yolk, the usual vehicle for pigments in the best kind of tempera-painting, must be regarded as essentially an oil-medium. As it dries, the oil hardens, and remains intimately commingled with the albuminous substances left behind on the evaporation of the water present. These albuminous substances coagulate and become insoluble in the lapse of time - a change greatly accelerated by the old practice of exposing the finished tempera picture to sunshine previous to varnishing it.