CH. xxv.J THE BLOOD. 405 



(e) Coagulative those which convert soluble into insoluble 

 proteids. Examples : rennet, fibrin ferment, myosin ferment. 



Most ferment actions are hydrolytic i.e., water is added to 

 the material acted on, which then splits into new materials. 

 This is seen by the following examples : 



1. Conversion of cellulose into carbonic acid and marsh gas 

 (methane) by putrefactive organisms 



(C 6 H 10 5 )n + nH 2 = 3 nCO, + 3 nCH 4 



[Cellulose] [Water] [Carbonic [Methane] 

 acid] 



2. Inversion of cane sugar by the unorganised ferment 

 invertin 



CjjHjjOj, + H 2 = C 6 H 12 6 + CgH 1; ,0 6 



[Cane sugar] [Water] [Dextrose] [Levulose] 



A remarkable fact concerning the ferments is that the sub- 

 stances they produce in time put a stop to their activity ; thus, 

 in the case of the organised ferments, the alcohol produced by 

 yeast, the phenol, cresol, &c., produced by putrefactive organisms 

 from proteids, first stop the growth of and ultimately kill the 

 organisms which produce them. In the case of the enzymes 

 also the products of their activity hinder and finally stop 

 their action, but on the removal of these products the ferments 

 resume work. 



This fact suggested to Croft Hill the question whether ferments 

 will act in the reverse manner to their usual action : and in the 

 case of one ferment, at any rate, he finds this to be the case. 

 Inverting ferments, as we have just seen, usually convert a 

 disaccharide into monosaccharides. One of these inverting 

 ferments, called maltose, converts maltose into dextrose. If, 

 however, the ferment is allowed to act on strong solutions of 

 dextrose, it converts a small proportion of that sugar back into 

 maltose again. 



Ferments act best at a temperature of about 40 C. Their 

 activity is stopped, but the ferments are not destroyed, by cold ; 

 it is stopped and the ferments killed by too great heat. A 

 certain amount of moisture and oxygen is also necessary ; there 

 are, however, certain micro-organisms that act without free 

 oxygen, and are called anaerobic in contradistinction to those 

 which require oxygen, and are called aerobic. 



The chemical nature of the enzymes, or unorganised ferments, 

 is very difficult to investigate; they are substances that elude 

 the grasp of the chemist to a great extent. So far, however, 



