In all living bodies we find that the protoplasm is of a more or less albuminous nature.

Albuminous substances possess a very complex inter-molecular grouping, and very high atomic weights. Many different forms are found in animals, and along with albumins we must associate bodies like mucin, which probably have a very important relation to it, inasmuch as a body nearly, if not quite, identical with mucin forms the nucleus of the red blood-corpuscles in fowls,1 and a substance of an allied nature also occurs in the circulating fluid which represents the blood in the echino-dermata.2 The albumin of serum may be taken as a representative of such substances; it is soluble in water, but, at a certain temperature, is coagulated and precipitated. It is coagulated also by alcohol, but if the coagulum is quickly placed in water it redissolves; if allowed to remain for some time exposed to the action of the alcohol it becomes permanent and insoluble. An insoluble precipitate also falls on the addition of tannic acid, both lead acetates, and mercuric chloride. The reagents just mentioned precipitate all the albumins, even from somewhat dilute solutions; in strong solutions precipitates are also formed by silver nitrate, copper sulphate, and zinc chloride.

When these are added to albumin containing only a small quantity of water, as, for example, the white of an egg, they form with it a solid mass of albuminate. A small quantity of strong potash added to the white of egg produces a solid transparent jelly of albuminate of potash, and a similar but opaque jelly is formed by the use of caustic lime or baryta in the place of potash : these albuminates are, however, soluble in water.

Albumin dissolves in alkalies, and may be partly precipitated by neutralising. The alkaline solution is not coagulated by heat, and, in fact, the substance present in the solution is no longer serum albumin, but a compound of the albumin with the alkali, or alkali-albuminate.

1 Lauder Brunton after Kuhne, Journ. of Anat. and Physiol. Nov. 1869.

2 Schafer, Proc. Boy. Soc, vol. xxxiv., p. 370.

Albumin is precipitated by a small quantity and dissolved by excess of most mineral acids, forming with them acid-albu-minates; thus a watery solution of albumin is precipitated by concentrated nitric, sulphuric, or hydrochloric acid. It is also precipitated by acetic acid along with a considerable quantity of a neutral salt of an alkali or alkaline earth, or of gum arabic or dextrin. This precipitation is perhaps best marked with nitric acid, but it only occurs with moderate quantities of nitric acid. When a minute quantity only of the acid is added, no precipitation takes place, and the solution remains clear; but a nitric-acid-albuminate containing a small quantity of acid is formed, and if the solution is now boiled no coagulum will form. On the addition of more acid, however, a second nitric-acid-albuminate, insoluble in water, is produced, and a precipitate falls. On the addition of more acid still, the precipitate is redissolved, and a third nitric-acid-albuminate is formed, soluble in water, and not precipitated on boiling.

The temperature at which albumin coagulates is altered by acids and alkalies. Alkalies generally tend to raise the temperature of coagulation, and when added in large quantities prevent it altogether.

Very dilute acetic and phosphoric acid, on the other hand, tend to lower the coagulating point, although large quantities may interfere with coagulation.

Neutral salts, such as sodium chloride or sulphate, also lower the coagulating point.

The organic alkaloids which have such a powerful action on the animal body appear to resemble acids rather than alkalies in their effect upon albumin, because, according to Rossbach, they lower considerably instead of raising the point of coagulation.

Albumin undergoes an extraordinary change in consequence of the action of ozone, and becomes, after exposure to it, un-coagulable by boiling, and by acids, excepting in large quantities, and by metallic salts, with the exception of basic acetate of lead, and of alcohol.

The action of alkaloids upon this ozonised albumin is even more remarkable than upon ordinary albumin, for when mixed with it in small quantity, they restore its coagulability to the albumin, and cause it to coagulate far under the boiling-point. When added to the albumin before exposure to a stream of ozone, they prevent the albumin being altered by it, in the way which it would otherwise be, and it remains coagulable by heat, in the same way as if it had not been exposed to the action of ozone at all. It is therefore evident that the alkaloids not only increase the coagulability of ordinary albumin at a high temperature, but that they act upon it at ordinary temperatures (30°-40° C.) and destroy its affinity for ozone. This action will naturally interfere with the processes of oxidation in protoplasm; but the methods of examining this action will be described later on (p. 69).

When a solution of pure albumin is added to a mixture of guaiac and vegetable protoplasm, it greatly lessens the blue colour, which would otherwise be produced. The cause of this appears to be that albumins or albuminous substances have such an affinity for ozone that they take it up instead of allowing it to act on the guaiac. This affinity for ozone is diminished by the action of alkaloids.

This is shown by taking several tubes containing an albuminous solution of a certain strength. Reserving one as a standard, the alkaloids are added to the others, and after a certain time has elapsed, so as to allow the alkaloid to affect the albumin, a small quantity of lettuce water is mixed with each, and then a little guaiac. In the standard one the colour will be least, because the albumin not having been acted upon by the alkaloids will interfere with the reaction of the lettuce water and the guaiac upon each other. In the others a blue colour will appear with greater or less intensity, according as the albumin has been more or less affected by the alkaloid. This experiment, however, is not free from fallacy, because there is to be considered not merely the action of the alkaloid upon the albumin, but its action on the protoplasm as well, and it is therefore advisable to use it in a quantity which is small as compared with the amount of albumin employed.1