392 DIGESTION OF PROTEIDS. [BOOK n. 



If any other form of coagulated albumin (e.g. precipitated 

 acid- or alkali- albumin, suspended in water and boiled) be treated 

 in the same way, a similar solution takes place. The readiness 

 with which the solution is effected, will depend, cceteris paribus, 

 on the smallness of the pieces, or rather on the amount of surface 

 as compared with bulk, which is presented to the action of the juice. 

 The solution thus obtained, and rendered clear by nitration, pre- 

 sents two marked features ; it is not coagulated by boiling, and the 

 precipitation which it gives on neutralisation is variable in quantity, 

 being sometimes exceedingly small. Yet the ordinary tests for 

 proteids (15) shew that it contains a large amount of proteid in 

 solution, and indeed the addition to the solution of an adequate 

 quantity of alcohol will throw down a body, which both tests and 

 elementary analysis shew to be a proteid. But this proteid differs 

 from all the proteids mentioned above in that, on the one hand, 

 it is soluble in distilled water, without the addition of acids, 

 alkalis or neutral salts, and on the other hand its solutions are not 

 coagulated by heat. It is a form of proteid still more soluble 

 than any of those which we have hitherto studied. 



Gastric juice then readily dissolves coagulated proteids, which 

 otherwise are insoluble, or soluble only, and that with difficulty, in 

 very strong acids ; and in doing so, converts them into an ex- 

 ceedingly soluble form of proteid. 



When proteids, which are soluble in water or in dilute acid, 

 are treated with gastric juice, no visible change takes place ; 

 but nevertheless, it is found on examination that the solutions 

 have undergone a remarkable change, the nature of which is 

 easily seen by contrasting it with the change effected by dilute 

 acid alone. If raw white of egg, largely diluted with water 

 and strained, be treated with a sufficient quantity of dilute 

 hydrochloric acid, the opalescence or turbidity which appeared 

 in the white of egg on dilution (and which is due to the 

 precipitation of various forms of globulin accompanying the 

 egg-albumin in the raw white) disappears, and a clear mixture 

 results. If a portion of the mixture be at once boiled, a large 

 deposit of coagulated albumin occurs. If, however, the mixture 

 be exposed to 50 or 55 C. for some time, the amount of coagulation 

 which is produced by boiling a specimen becomes less, and, finally, 

 boiling produces no coagulation whatever. By neutralisation, 

 however, the whole of the albumin (with such restrictions as the 

 presence of certain neutral salts may cause) may be obtained in 

 the form of acid-albumin, the filtrate after neutralisation containing 

 no proteids at all (or a very small quantity). Thus the whole of 

 the albumin present in the white of egg may be, in time, converted, 

 by the simple action of dilute hydrochloric acid, into acid-albumin. 

 Serum-albumin similarly treated undergoes, in course of time, a 

 similar conversion into acid-albumin, and we have already seen 

 ( 59) that solutions of myosin or of any of the globulins are with 



