ALBUMINOUS MATTERS. 81 



Another characteristic feature of the proximate principles of this 

 group is their property of coagulation. Those which are naturally fluid 

 suddenly assume, under certain conditions, a solid or semi-solid consist- 

 ency. They are then said to be coagulated ; and, when once coagulated, 

 they cannot usually be made to resume their original condition. This 

 property of coagulability is not only a marked quality of the albu- 

 minous matters as a class, but it often serves to distinguish them 

 from each other by the different special conditions under which it is 

 manifested by each one. Thus the substance producing fibrine coagulates 

 spontaneously on being withdrawn from the bloodvessels ; albumen, on 

 being subjected to the temperature of boiling water ; caseine, on being 

 placed in contact with an acid. When an albuminous substance thus 

 coagulates, the change which takes place is a peculiar one, and differs 

 from that by which a mineral salt is precipitated from its watery solu- 

 tion. The albuminous matter, in coagulating, appears to assume a 

 special condition, and to permanently change its properties; but, in 

 passing into the solid form, it still retains ^all the water with which it 

 was previously united. Albumen, when coagulated, retains the same 

 quantity of water in union with it which it held before. After coagu- 

 lation, this water may be driven off by evaporation, in the same manner 

 as previously ; and on being once more exposed to moisture, the organic 

 matter will again absorb the same quantity, though it will not assume 

 the liquid form. The coagulated substance may sometimes be dissolved 

 by certain chemical agents, as the caustic alkalies ; but it is not by 

 this means restored to its original condition. It rather suffers a still 

 further alteration. 



In many instances we are obliged to resort to coagulation in order 

 to separate an albuminous substance from the other proximate princi- 

 ples with which it is associated. This is the case, for example, with 

 the fibrine of the blood, which is obtained in the form of flocculi, by 

 beating freshly-drawn blood with a bundle of rods. But when separated 

 in this way, it is already in an unnatural condition, and no longer 

 represents exactly the original fluid fibrine as it existed in the circulating 

 blood. Nevertheless, this is the only mode in which it can be examined, 

 as there are no means of bringing it back to its previous condition. 



Another important property of the albuminous matters is that they 

 excite, in other proximate principles and in each other, those peculiar 

 indirect chemical changes which have been termed catalyses or catalytic 

 transformations. That is to say, they produce the changes referred to, 

 not directly, by combining with the substance which suffers alteration, 

 or with any of its ingredients ; but simply by their presence, which 

 induces the chemical change in an indirect manner. We do not under- 

 stand the manner in which these changes are accomplished, but the 

 influence thus exerted by the albuminous matters is a very marked one, 

 and is of great importance in many of the acts of animal and vegetable 

 nutrition. A comparatively small quantity of the catalytic body is 

 often capable of inducing a palpable change in a large quantity of 



