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the problem of whether there is not here some discontinuity in nature ; whether 

 the play of molecular and atomic forces occurring outside the living organism can 

 ever account for the whole of the complexity and correlation of chemical and 

 physical interactions demonstrable within the living structure. 



As yet we are of course far from any answer to this question, and no one in a 

 scientific assembly like this will call iipon us for prophecies. Yet the subject to 

 which I shall devote my Address has a bearing upon this question. I propose to 

 consider a particular aspect of the relation of chemical changes in a test-tube 

 to those taking place in a living growing plant, and this in the spirit of one who 

 craves for continuity throughout natural phenomena. 



The point of view from which the chemist regards the reaction taking place in 

 his test-tube has undergone a change in the last twenty years, a change bringing 

 it more into uniformity with that of the biologist. No longer content with an 

 equation as a final and full expression of a given reaction, the chemist now 

 studies with minutest detail and with quantitative accuracy the progressive 

 stages of development of the reaction ^ and the effect upon it of varied external 

 conditions, of light, temperature, dilution, and the presence of traces of foreign 

 substances. 



Perhaps it is too much to believe that this, as it were physiological, study of each 

 reaction is the eifect of some benign irradiation from the biological laboratory. 

 At least, however, it is true that it is the modern study of ' slow ' chemical 

 reactions which has made all this possible, and the living organism consists 

 almost entirely of slow reactions. The earliest studied chemical reactions, those 

 between substances which interact so quickly that no intermediate investigation 

 can be made, did not of course lend themselves to this work, but nowadays 

 whole classes of reactions are known which are only completed hours or days 

 after the substances are initially mixed. To the slow reactions belong all the 

 hydrolytic and dehydration changes of carbohydrates, fats, and proteids that bulk 

 so largely in the metabolism of plants and animals, together with other 

 fermentation changes such as are brought about by oxidases, zymases, and 

 enzymes in general. This precise quantitative study of chemical reactions has 

 been developing with remarkable acceleration for some twenty-five years, till it is 

 grown almost into an independent branch of science, physical chemistry. This 

 is sometimes called ' general chemistry ' because its subject is really the 

 fundamental universal laws of the rate of chemical change, and these laws hold 

 through all the families, genera, and species of chemical compounds, just as the 

 same physiological laws apply to all the different types of plants. 



Now if these laws are fundamental with all kinds of chemical change they must 

 be at work in the living metabolic changes. If the chemical changes associated 

 with protoplasm have any important factor or condition quite different from the 

 state of things which holds when molecules react in aqueous solution in a test- 

 tube, then it might happen that the operation of these principles of physical 

 chemistry would be obscured and not very significant, though it is inconceivable 

 that they should be really inoperative. 



My present intention, then, is to examine the general phenomena of 

 metabolism in an attempt to see whether the operations of these quantitative 

 principles are traceable, and if so how far they are instrumental in giving a clearer 

 insight into vital complexity. 



The Dominance of Ieeitability in Physiologt. 



I think that certain manifestations of these principles are indeed quite clear, 

 though not generally recognised, and that this neglect is largely due to the 

 dominance of what our German colleagues call ' Reizphysiologie ' — the notion 

 that every change in which protoplasm takes part is a case of the ' reaction ' of an 



' Modern research has made it clear that reactions conventionally represented by 

 complex equations of many interacting molecules really take place in a succession 

 of simple stages, in each of which, perhaps, only two molecules interact. 



