CHEMICAL BASIS OF THE ANIMAL BODY. 53 



spark. The fact of immediate interest in each of the above instances is 

 that a minute trace of the substance which determines the occurrence 

 of the reaction is able to produce change in an indefinitely large mass 

 of the other reagents without itself undergoing any final alteration. 

 Turning to the chemistry of animal and vegetable cells it is found that 

 in many cases substances may be extracted from them which possess 

 to an even more striking degree the property of inducing change in 

 an indefinitely large mass of certain other substances without them- 

 selves undergoing any observable alteration. These agents are known 

 as the enzymes or soluble ferments, and the essential conception of an 

 enzyme is summed up in the above statement of the most remarkable 

 characteristic of their activity. Further investigation of these enzymes 

 shows that their activity is dependent upon many subsidiary factors 

 which are more or less common to them all. Thus their activity is 

 largely dependent upon temperature, being absent at sufficiently low 

 temperatures, increasing as the temperature is raised to a certain 

 optimal point which varies slightly for different enzymes, then again 

 diminishing as the temperature is further raised and finally dis- 

 appearing. By the action of a sufficiently high temperature they 

 permanently lose their characteristic powers and are now spoken 

 of as being 'killed.' Again the enzymes are extremely sensitive to 

 the reaction, whether acid, alkaline or neutral, of the solutions in 

 which they are working, also to the presence or absence of various 

 salts, some of which merely inhibit their action while others perma- 

 nently destroy it ; and their activity is in all cases lessened and finally 

 stopped by the presence of an excess of the products to whose formation 

 they have given rise. It has been already said that an enzyme may 

 be killed by exposure to a high temperature, but this only holds good 

 when they are in solution, or if in the solid form they are heated in a 

 moist condition. When perfectly dry they may be heated to 100 160 

 without any permanent loss of their powers 1 . It will be seen that so 

 far the enzymes have been characterised solely with reference to the 

 peculiarity of their mode of action and to the influence of surrounding 

 conditions upon that activity, and the question of their probable 

 chemical composition has been left untouched. Notwithstanding the 

 frequent endeavours which have been made to prepare the enzymes in 

 a pure condition, it is unwise to lay any great stress upon the results 

 of the analysis of these so-called 'pure ferments,' bearing in mind 

 that, as in the case of the proteids, no criterion of their purity exists. 

 This much however may be said. In the majority of cases, analysis 



1 Hiifner, Jn. f. prakt. Chem. Bd. v. (1872), S. 372. Al. Schmidt, Centralb. /. d. 

 med. Wiss. 1876, S. 510. Salkowski, Virchow's Arch. Bd. LXX. (1876), S. 158; LXXXI. 

 (1880), S. 552. Hiippe, Mittheil. d. Kaiserl. Gesundheitsamtes, i. 1881. 



