ALBUMINOUS COMPOUNDS. 53 



we shall notice the Inorganic Constituents, which, with the preceding, enter into 

 the composition of the Human fabric. 1 



1. Albuminous Compounds. 



18. Under this head may be grouped together a series of organic compounds, 

 which are of primary importance in histogenesis or the formation of tissue ; and 

 this not less in the Vegetable kingdom than in the Animal ; the original cell- 

 walls of the Plant, as well as those of the Animal, being formed at their ex- 

 pense. 2 Putting aside those Albuminous substances which are proper to the 

 Vegetable kingdom, and restricting ourselves to those which are parts of the 

 Animal body or are formed within it, we find this group to consist of Albumen, 

 which may be taken as its type, with Casein, Globulin, and Fibrin; these are 

 also generally known under the designation of " Protein-compounds;" 3 and the 

 following properties are for the most part common to them all : They occur in 

 two conditions, namely, in a soluble, and in an insoluble or scarcely soluble state j 

 it is in the former condition that we find them naturally existing in the animal 

 fluids. The soluble modification, when dried, forms a faint yellow, translucent, 

 friable mass, having no smell or peculiar taste ; it dissolves in water, but is in- 

 soluble in alcohol and ether ; it is precipitated by alcohol from the aqueous 

 solution, after which it is usually insoluble in water. The aqueous solution is 

 precipitated by most metallic salts, and the precipitate generally contains the 

 acid and base of the salt employed, in addition to the protein-compound. The 

 greater number cannot be precipitated from their aqueous solution by alkalies, 

 or by most of the vegetable acids ; but they are precipitated by mineral acids 

 (with the exception of ordinary phosphoric acid) and by tannic acid ', and by 

 some of these they are converted into their insoluble form. Into this, more- 

 over, the greater number of them are changed by boiling ; and it is in this 

 mode that their insoluble forms are commonly obtained. The insoluble com- 

 pounds, when dried, are white and pulverizable, without taste or smell, without 

 reaction on vegetable colors, and insoluble in water, alcohol, ether, and all 

 indifferent menstrua ; but they are all more or less readily dissolved by alkalies, 

 from which they may be precipitated by mere neutralization with acids. Their 

 behavior towards different acids is by no means so uniform, and must be sepa- 

 rately described in each case. All of them, however, are acted on in a peculiar 

 manner by concentrated Nitric and Hydrochloric acids ; the former giving them 

 when heated a deep lemon-colored tint ; whilst the latter causes them to 

 assume a gradually-increasing Hue color, which becomes intense when they 

 are exposed to warmth and air. Again, they are all dissolved by concentrated 

 acetic acid and other organic acids, as well as by phosphoric acid ; and are 

 precipitated from these solutions by ferrocyanide of potassium. All the protein- 

 compounds contain Sulphur, which seems to be one of their essential constitu- 

 ents, not being capable of entire separation without the complete destruction of 

 the organic substance, although a part may be withdrawn by digestion with 

 fixed alkalies. 



1 In the following outline, the authority principally relied upon will be the "Physiolo- 

 gical Chemistry" of Prof. Lehmann, of which a translation (by Prof. G. E. Day) is now 

 in course of publication by the Cavendish Society. 



2 See "Principles of Physiology, General and Comparative," Am. Ed., $ 136. 



3 Although this designation was first given them by Mulder under an idea (which has 

 since proved to be erroneous) that he could obtain from either of them a certain organic 

 base, free from sulphur and phosphorus, to which he gave the name of Protein, yet it has 

 been found to be so convenient both in Chemistry and Physiology, that there seems to be 

 a general accordance in its retention. See Lehmann's "Physiological Chemistry," vol. i. 

 p. 326. 



