October i, 1908 J 



NA TURE 



557 



phuf in a living growing plant, and (hi;; in the spirit of 

 one who craves for continuity throughout natural pheno- 

 mena. 



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 e.xpression of 

 a iii\«_-n reaction, the chemist now studies with minutest 

 det.-'ll and with quantitative accuracy the progressive stages 

 of development of the reaction ' and the effect upon it of 

 varied e.'iiernal 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 effect 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 re- 

 actions. The earliest studied chemical reactions, those 

 between substances which interact so quickly that no inter- 

 mediate investigation can be made, did not of course lend 

 themselves to this work, but nowadays whole classes of 

 reactions a>e known which are only completed hours or 

 d.ivs 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, zvmases, and enzymes in general. This precise 

 quantitative study of chemical reactions has been develop- 

 ing with remarkable acceleration for some twenty-five 

 vears, until 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 physio- 

 logical 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 meta- 

 bolic changes. If the chemical changes associated with 

 protnplasm have any important factor or condition quite 

 different from the state of things which holds when mole- 

 cules react in aqueous solution in a test-tube, then it 

 might happen that the operation of these principles of 

 ph\sical chemistry would be obscured and not very sig- 

 nificant, 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 trace- 

 able, and if so how far they are instrumental in giving a 

 clearer insight into vital complexity. 



The D0MIN.AXCE OF Irrit.^bilitv in Phvsiologv. 



I think that certain manifestations of these principles 

 are indeed quite clear, though not generallv 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 '\irritable " living sub- 

 stance to a " stimulus." Now this general conception of 

 protoplasmic irritabilitv. of stimuli and reactions was, of 

 course, a splendid advance, the earlv development and ex- 

 tension of \vhich we ow-e largely to our veteran physiologist 

 Prof. Pfeffer, of Leipzig. Great as is the service it has 

 rendered to many departments of botany, yet in one direc- 

 tion, I think, it has overflowed its legitimate bounds and 

 swamped the development of the physical-chemical con- 

 cepts which I shall indicate later on. The great merit of 

 the " stimulus and reaction " conception is that it supplies 

 ;i very elastic general formula for the sort of causal con- 

 nection that we find occurring in all departments of 

 biology ; a formula which allows the phenomena to be 

 grouped, investigated, and formally expounded, whether 

 they be the temporary turgor-movements of " sensitive " 



1 Rrodern research has made it clear that reactions cnnvenlionally repre- 

 sented by complex equations of many interacting molecules really take place 

 in a succesFion of simple stages, in each of which, perhaps, only two 

 molecules interact. 



plants, the permanent growth movements of tropistic curva- 

 tures, ' or the complex changes of plant-form and develop- 

 ment' that result from present and past variations of ex- 

 ternal conditions. 



The strength and the weakness of the conception lie in 

 its extraordinary lack of particularity. When an irritable 

 cell responds to a stimulus by a reaction nothing is imphed 

 about the mechanism connecting the cause and the effect, 

 and nothing even about the relative magnitudes of these, 

 but all this is left for special research on the case under 

 consideration. The one natural chain of cause and effect 

 that is recognised to be outside this comprehensive category 

 is that rather uncommon one in which a definite amount 

 of energy of one kind is turned into an equivalent definite 

 amount 'of energy of another. Here we have a direct 

 " equation of energy," whereas in a reaction to a stimulus 

 we are said to have typically an " unloosing " effect — a 

 liberation of potential energy by a small incidence of 

 outside energy, as in the classical analogies, drawn from 

 completely comprehended non-living things, of a cartridge 

 exploded by a blow, or the liberation into action of a head 

 of water bv the turning of a tap. 



So elastic a conception may be easily stretched to ht 

 almost any sequence of phenonicna with the apparent close- 

 ness that argues a bespoken garment. We must therefore 

 be critically on our guard against cases of such sartorial 

 illusion. 



The Principles of Chemical Mech.inics. 



That mv consideration of particular cases may be in- 

 telligible it seems necessary that I devote a few minutes 

 to outlining the four quantitative mechanical principles 

 which govern every single chetnical reaction, though much 

 that I have to say has been drawn from elementary books- 

 on physical chemistrv. 



These four principles are concerned with (i) the nature 

 of the reaction in question ; (2) the amount of reacting, 

 substances that happen to be present ; (3) the temperattjre 

 at which the reaction is taking place; and (4) the m- 

 fiuence of catalysts upon the reaction. 



For the moment we will confine ourselves to the first 

 two matters, and assume that catalysts are absent and 

 the substances at constant temperature. 



(i) The first principle that we have to consider is that 

 which declares that no chemical reaction is really in- 

 stantaneous, though the interaction of substances is often 

 so fast that a direct measurement of its rate cannot be 

 made • and, further, that every reaction has its own specific 

 rearli'on-velocitv which distinguishes it from other reactions. 

 This is expressed bv giving to each particular reaction a 

 numerical velocity-coefficient which is low or high pro- 

 portionally as the reaction is slow or quick. 



(2) This coefficient only expresses the actual experimental 

 velocity w-hen the reacting substances are present in unit 

 concentration, because difference of concentration is just the 

 most important factor controlling the actual reaction- 

 velocity. 



If a' solution of a substance .\ of unit concentration is 

 undergoing change, then to keep this reaction going at 

 its present rate fresh ainounts of A must be added con- 

 tinually just to equal the amount removed by the reaction 

 and so' keep the substance up to unit concentration. The 

 amount of A that had to be added thus per unit time would 

 give an exact measure of the amount being decomposed, 

 X.e., of the soecific velocity of this reaction. 



If the reaction were started with a at double unit con- 

 centration, then twice as much A would have to be added 

 per unit time to keep the reaction velocity constant at the 

 double rate it w^ould have started at. 



And with higher concentrations proportionally more a 

 would have to be added. It is therefore shown that the 

 amount of chemical change going on in unit time is pro- 

 portional to the concentration. This is a most fundamental 

 principle of chemical mechanics, known as trie law of mass, 

 and it may be stated thus : the amount of chemical change 

 taking place at any time is always proportional to the 

 amount of actively reacting substance {or substances) 

 present. 



To carry out experiments by the procedure given above 

 is in practice very difficult, and the velocities of reactions 

 are never measure'd by the chemist in this way. In a living 

 organism this continual bringing up of new supplies of 



NO. 2031, VOL. 78] 



