338 Journal of Agriculture. [8 June, 1907. 



If pure copper be dipped into pure nitric acid no reaction is at first 

 observed, but introduce an unweighably small amount of sodium nitrite 

 and the copper is immediately attacked bv the acid. Catalytic reactions 

 are checked by cold and increased bv heat and show a marked sensi- 

 tiveness to the presence of certain other bodies for which the term poisons 

 has been borrowed from medical science. Thus a minute amount of 

 prussic acid will stop the catalytic action of platinum on hydrogen per- 

 oxide and mere traces of arsenic will hinder the catalytic action of the 

 same metal as it is emploved in the manufacture of sulphuric acid. 



Xow this idea can be extended to the ferments of saliva and gastric 

 juice. The action of the diastase of saliva or of malt extract is simply 

 an acceleration of the change from starch into sugar which is normally 

 progressing at a very slow rate. The same may be said of pepsin. It 

 has been noted in the previous chapter that proteins tend to disintegrate 

 even when sheltered from bacteria, but that some vears must elapse before 

 this is noticeable. Now all that pepsin does is to bring about in an 

 hour or two what might otherwise take a full century to accomplish. 

 Neither the diastase nor the pepsin does any actual work ; their action may 

 be compared with oiling the axles or removing the brake from a truck 

 which is slowly crawling downhill. To catalvsors such as diastase and 

 pepsin, which are products of life but not themselves living, the term 

 ENZYMES has been given. 



But what of the fermentations like the alcoholic which apparently 

 demand the existence of life in their midst? In 1895 a discovery was 

 made that yeast may be killed and an extract made from it w^hich can 

 carry out the alcoholic fermentation in a rapid manner. It had alreadv 

 been shown that sugar solution placed in sunlight shows traces of alcohol 

 after a lapse of a considerable time. Clearly the yeast when it trans- 

 forms sugar into alcohol does so by means of an enzyme which 

 accelerates a process already in action. Kindred discoveries were made 

 with various bacteria and with living organs so that we can now state 

 that a large number of the reactions carried out in the animal body are 

 due to enzymes, which enzymes are specially made by living cells for 

 those particular reactions. 



When we consider the immense number of chemical substances which 

 are made or transformed bv living things we cannot but be amazed at 

 the ease with which these activities are conducted and in the face of so 

 many difficulties. A chemist in his laboratorv can employ high and low 

 temperatures, high and low pressures, he may use strong acids and 

 alkalies, he may crystallise, volatilise and sublime, yet even in the mam- 

 malian body, highly as it is organised, all reactions proceed within a 

 temperature range of a few degrees, pressure is everywhere constant, 

 the reaction is neutral or only very faintly acid or alkaline, crystalliza- 

 tion, at least in animals, is never resorted to, and volatilisation and sub- 

 limation are impossible. Despite this heavy handicap the body can pro- 

 duce, rapidly, accurately and economically, a host of chemical com- 

 pounds which defy the utmost resources of the chemist even to analyse. 

 As research in physiology has proceeded it has been found that many 

 chemical reactions which were formerly thought to be due to mysterious 

 powers of life can be classified as catalytic. The mysterious part remains 

 in the fact that the cell makes anew the proper catalysor at the proper 

 time and in the proper place. This is but another response to change in 

 environment. 



