The form in which protein exists in hair, horn, nails, and the epidermis, and called by Simon keratine, has been but imperfectly ex amined. That these substances are composed chiefly of protein is proved by the circumstance that if a solution of them be made in caustic potash and neutralized with acetic acid, a co pious precipitate of protein is thrown down. It is probable that other modifications of protein will hereafter be found to exist in the animal body, but those which I have now described are all which have hitherto been detected.
The animal body, however, is not the only source from which protein and its compounds are to be obtained. The researches of modern chemists have led to the interesting fact that they exist in the vegetable kingdom also, and that they are there so extensively disseminated that not a leaf, a seed, or a twig, in any of the various tribes of plants, is free from them; and it is highly probable that the whole of the protein compounds constituting the bodies of anitnals are derived from plants. In the pre sent state of analysis it is perhaps too much to say that the forms in which we find protein in vegetables are absolutely the same, with regard to the minute quantities of sulphur and phos phorus, as those found in animals ; but as far as we are able to judge from similarity of pro perties, we may safely divide them in the same way as the analogous animal principles ; viz. into vegetable fibrin, vegetable albunzen, and vegetable casein. They all yield, when heated with strong hydrochloric acid, blue or purple solutions ; and when they are digested with a solution of potash, and neutralized with acetic acid, protein is invariably produced.
Vegetable fibrin is found most abundantly in the seeds of the cerealia, as wheat, oats, &c.: it is also found dissolved in the juice of most plants, especially that of grapes, carrots, tur nips, and beetroot, from which it shortly sepa rates in the form of a flocculent precipitate when taken from the plant and allowed to stand. The readiest way of preparing it is to knead wheaten flour into a paste with water, and then wash it on a linen cloth IA ith a stream of cold water until the whole of the starch is removed, which is known by the water passing through quite clear : the viscous mass which remains on the cloth is subsequently purified by washing with alcohol and ether, in both of which the fibrin is insoluble. When dry it is a hard horny-looking substance, semitranspa rent, without taste or smell, and sufficiently heavy to sink in water, in which it is insoluble. Phosphoric and acetic acids readily dissolve it ; and it is reprecipitated in the form of white flocks from its acid solution by carbonate of ammonia and ferrocyanide of potassium, and yellowish by tincture of galls ; it is also preci pitated by bichloride of mercury and some other metallic salts. It is perfectly soluble in
solution of potash even when very dilute, and if the quantity of fibrin dissolved be large, the liquid loses its alkaline flavour.
Vegetable albumen is found to exist very abundantly in the juices of most plants, and still more so in nuts, almonds, and other oily seeds, where it is usually associated with ca sein. It may be easily recognized by boiling the expressed juice of any of the common cu linary vegetables after the fibrin has separated, when it coagulates in a manner similar to ani mal albumen. It may be obtained in a tole rably pure state by boiling the filtered juice of any of the leguminosce, and washing the preci pitate with alcohol and ether. It closely re sembles animal albumen in properties, and is distinguished from vegetable fibrin by its so lubility in water, and from vegetable casein by coagulating when heated.
Vegetable casein has also been called legu mine, from the circumstance of its being found most abundantly in the leguminosce, though it is by no means confined to that tribe of plants : it is also present in considerable quantity in company with albumen in most of the oily seeds, and in the juices of most nutritious vegetables. It may be obtained by the follow ing process. Peas or beans should be soaked in moderately warm water for some hours until they are sufficiently soft to allow of their being mashed in a mortar : the pasty mass is then mixed with a large quantity of water, which dissolves the casein, and thrown upon a cloth to filter. The starch passes through the filter together with the solution of casein, and if allowed to stand, gradually subsides to the bottom : when the liquid is clear, it is decanted by means of a syphon, and slightly supersa turated with acetic acid, which determines the precipitation of the casein in an impure state, but readily purified by washing with alcohol and ether. Vegetable casein resembles that obtained from milk in most of its properties ; gives the same insoluble skin when heated in contact with the air ; and is precipitated frona its aqueous solution of alcohol and several of the metallic oxides : it is also thrown down by both vegetable and mineral acids, redissolving in an excess of the former, except the acetic, and insoluble in excess of the latter. If a so lution of casein be allowed to stand some time, lactic acid is gradually formed, which causes it to coagulate, and putrefaction then begins, which, if any sugar is present, determines in it the alcoholic fermentation.