18 PROTEIDS OF SERUM. [BOOK i. 



neutral saline solutions and in dilute acids and alkalis, but is 

 easily dissolved in strong acids and alkalis. In the process of 

 solution it becomes 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 ; it is 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 

 precipitated form, and suspended in distilled water, it readily 

 dissolves into a clear solution upon the addition of a small quan- 

 tity 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- 



