THE CLOTTING OF BLOOD. 29 



a very weak solution of cupric sulphate, a violet or pink color which deepens 

 on heating. These are artificial reactions, not throwing much if any light 

 on the constitution of proteids ; but they are useful as practical tests enabling 

 us to detect their presence. ' 



The several members of the proteid group are at present distinguished 

 from each other chiefly by their respective solubilities, especially in various 

 saline solutions. Fibrin is one of the least soluble ; it is insoluble in water, 

 almost insoluble in dilute neutral saline solutions, and very sparingly soluble 

 in more concentrated neutral saline solutions and in dilute acids and alkalies. 

 In strong acids and alkalies it dissolves, but in the process becomes com- 

 pletely changed into something which is no longer fibrin. In dilute acids it 

 swells up and becomes transparent, but when the acid is neutralized returns 

 to its previous condition. When suspended in water and heated to 100 C., 

 or even to 75 C., it becomes changed, and still less soluble than before ; it is 

 said in this case to be coagulated by the heat, and, as we shall see, nearly all 

 proteids have the property of being changed in nature, of undergoing 

 coagulation and so becoming less soluble than before, by being exposed to a 

 certain high temperature. 



Fibrin, then, is a proteid distinguished from other proteids by its smaller 

 solubility ; it is further distinguished .by its peculiar filamentous structure, 

 the other proteids when obtained in a solid form appearing either in amor- 

 phous granules or, at most, in viscid masses. 



16. We may now return to the serum. 



This is perfectly fluid, and remains fluid until it decomposes. It is of a 

 faint straw-color, due to the presence of a special pigment substance, differing 

 from the red matter which gives redness to the red corpuscles. 



Tested by the xanthoproteic and other tests it obviously contains a large 

 quantity of proteid matter, and upon examination we find that at least two 

 distinct proteid substances are present in it. 



If crystals of magnesium sulphate be added to serum and gently stirred 

 until they dissolve, it will be seen that the serum as it approaches saturation 

 with the salt becomes turbid instead of remaining clear, and eventually a 

 white amorphous granular or flocculent precipitate makes its appearance. 

 This precipitate may be separated by decantation or filtration, washed with 

 saturated solutions of magnesium sulphate, in which it is insoluble, until it 

 is freed from all other constituents of the serum, and thus obtained fairly 

 pure. It is then found to be a proteid body, distinguished by the following 

 characters among others : 



1. It is (when freed from any adherent magnesium sulphate) insoluble in 

 distilled water ; it is insoluble in concentrated solutions of neutral saline 

 bodies, such as magnesium sulphate, sodium chloride, etc., but readily soluble 

 in dilute (e. g., 1 per cent.) solutions of the same neutral saline bodies. 

 Hence from its solutions in the latter it maybe precipitated either by adding 

 more neutral saline substance or by removing by dialysis the small quantity 

 of saline substance present. When obtained in a precipitated form, and 

 suspended in distilled water, it readily dissolves into a clear solution upon 

 the addition of a small quantity of some neutral saline body. By these 

 various solutions and precipitations it is not really changed in nature. 



2. It readily dissolves in very dilute acids (e. g., in hydrochloric acid even 

 when diluted to far less than 1 per cent.), and it is similarly soluble in dilute 

 alkalies, but in being thus dissolved it is wholly changed in nature, and the 

 solutions of it in dilute acid and dilute alkalies gives reactions quite different 

 from those of the solution of the substance in dilute neutral saline solutions. 

 By the acid it is converted into what is called acid-albumin, by the alkali 

 into alkali-albumin, both of which bodies we shall have to study later on. 



