18 PROTEIDS OF SERUM. [BOOK i. 



neutral saline solutions and in dilute acids and alkalis. In strong 

 acids and alkalis 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 amorphous 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 colour, 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, &c., but readily soluble in dilute (e.g. 1 p.c.) 

 solutions of the same neutral saline bodies. Hence from its 

 solutions in the latter it may be precipitated either by adding 

 more neutral saline substance or by removing by dialysis the small 

 quantity of saline substance present. When obtained in a pre- 

 cipitated 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 hydro- 



