ALBUMENOID SUBSTANCES. 75 



noid substances from other matters with which they are mingled. 

 Thus if a solution containing albumen, glucose, and sodium chloride 

 be immersed in a vessel of pure water, with a partition of parchment 

 paper between the two liquids, the glucose and the salt will pass 

 through the membrane and become diffused in the water, while the 

 albumen will remain behind. By renewing the water in the exte- 

 rior vessel and thus keeping up the activity of diffusion, nearly 

 the whole of the glucose and the salt may be removed from the 

 interior solution, and the albumen left in a purified condition. This 

 method is termed "dialysis," and is frequently employed for obtain- 

 ing albumenoid matters free from admixture with other substances. 

 There is one remarkable exception, among the albumenoids, to the rule 

 of non-diffusibility. It is that of "peptone," the substance produced, 

 from albuminous matters by digestion, which retains all their other 

 essential properties, but has acquired the power of passing by diffu- 

 sion through animal membranes. It is thus rendered capable of 

 being absorbed by the intestine, and of entering the current of the 

 circulation. 



The albumenoid substances are coagulable. This property consists 

 in their capacity, when fluid, of suddenly changing, under certain phys- 

 ical or chemical influences, to the solid form ; so that they either sepa- 

 rate from the other ingredients of the liquid, or convert the whole into 

 a gelatinous mass. This difference depends on the relative quantity in 

 which they are present, and on the manner in which they are associ- 

 ated with the other ingredients. Thus if a specimen of slightly 

 albuminous urine be heated, the albuminous matter is thrown down 

 as a flaky deposit, while the rest remains liquid ; but if the serum 

 of blood be treated in the same way it solidifies into a uniform 

 mass. In neither case is the process of coagulation a simple precipita- 

 tion, as where a mineral salt is thrown down from solution in a fluid. 

 The albumenoid substance, when coagulated, still retains its normal pro- 

 portion of water, and in the instance of the serum of blood, it holds 

 the whole of the water present, so that no liquid separates from the 

 coagulum. It may be driven off by evaporation, but the coagulated 

 albuminous matter is still hygroscopic, and will again take up water by 

 absorption to the same extent as before. A coagulated substance is 

 usually permanently altered in character, and cannot be restored to 

 fluidity except by means of acid or alkaline solvents, which still further 

 modify its original properties. 



Different albumenoid matters are coagulated by different agents, and 

 these reactions form a convenient and sometimes the principal test by 

 which they are distinguished. Thus albumen is coagulated by heat 

 or the mineral acids, but not by organic acids. Caseine, the albumi- 

 nous ingredient of milk, is coagulated by either mineral or organic 

 acids, but not by heat. The animal matter of the pancreatic juice is 

 coagulated by heat or the mineral acids ; but also by magnesium sul- 

 phate, which does not affect albumen. The coagulation of some albu- 



