The materials described in the present chapter owe their peculiar properties - at least, in great measure - to the presence of chemical compounds which contain the element nitrogen. Now, this element is not a constituent of any of the artists' materials already described, nor, indeed, of any others, except a few pigments, such as aureolin, Prussian blue, and indigo. The presence of nitrogen in an organic compound is very often accompanied by a measure of instability, or proneness to change; the nitrogenous constituents of eggs, and of size, afford illustrative examples. Another source of weakness in the composition of the nitrogenous constituents, both of the white and of the yolk of eggs, lies in the presence of another element - namely, sulphur. Part of this sulphur readily leaves the original substance, yielding simpler compounds, such as sulphuretted hydrogen, and ammonium sulphide, which possess the objectionable property of discolouring many of the metallic pigments used by artists. On the other hand, all these nitrogenous bodies are susceptible of coagulation, whereby they become insoluble, and very much less prone to change. Indeed, the majority of them may be turned into a substance which is virtually leather, a material which resists decay in the most marked manner.

This tanning operation may be readily effected by treating the substances in question with a solution containing tannin, the active ingredient of oak-bark, sumach, nut-galls, etc.

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

Yolk

White

Water - - - -

- 51.5 -

- - 84.8

Albumen, Vitellin, etc.

- 15.0 -

- - 12.0

Fat or Oil - - -

- 22.0 -

- - 0.2

Lecithin, etc - - -

- 9.0 -

- - trace

Mineral Matter - -

- 1.0 -

- - 0.7

Other Substances -

- 1.5 -

- - 2.3

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 per cent. 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 per cent. 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 per cent. 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 per cent. of it, and associated with this oil there is no less than 9 per cent. 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.