98 PROXIMATE PRINCIPLES OF THE THIRD CLASS. 



Another peculiar phenomenon characteristic of organic substances 

 is their coagulation. Those which are naturally fluid suddenly as- 

 sume, under certain conditions, a solid or semi-solid consistency. 

 They are then said to be coagulated ; and after coagulation they 

 cannot be made to resume their original condition. Thus fibrin 

 coagulates on being withdrawn from the bloodvessels, albumen on 

 being subjected to the temperature of boiling water, casein on being 

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

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

 no resemblance to the precipitation of a solid substance from a 

 watery solution. On the contrary, the organic substance merely 

 assumes a special condition ; and in passing into the solid form it 

 retains all the water with which it was previously united. Albumen, 

 for example, after coagulation, retains the same quantity of water in 

 union with it, which it held before. After coagulation, accordingly, 

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

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

 matter will again absorb the same quantity, though it will not re- 

 sume the fluid form. 



By coagulation, an organic substance is permanently altered ; and 

 though it may be afterwards dissolved by certain chemical re-agents, 

 as, for example, the caustic alkalies, it is not thereby restored to its 

 original condition, but only suffers a still further alteration. 



In many instances we are obliged to resort to coagulation in 

 order to separate an organic substance from the other proximate 

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

 with the fibrin of the blood, which is obtained in the form of floe- 

 culi, 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 and no longer represents exactly the original fluid fibrin, as it ex- 

 isted 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 organic substances is that 

 they readily excite, in other proximate principles and in each other, 

 those peculiar indirect chemical changes which are termed catalyses 

 or catalytic trarisformations. 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. 

 Thus, the organic substances of the intestinal fluids induce a cata- 



