880 * TRANSACTIONS OF SECTION K. 



' irritable ' living substance to a ' stimulus.'' Now this general conception of 

 protoplasmic irritability, of stimuli and reactions was, of course, a splendid 

 advance, the early development and extension of which we owe largely to our 

 veteran physiologist Professor Pfefler of Leipzig. Great as is the service it 

 has rendered to many departments of botany, yet in one direction, I think, it 

 has overflowed its legitimate bounds and swamped the development of the 

 physical-chemical concepts which I shall indicate later on. The great merit of 

 the ' stimulus and reaction ' conception is that it supplies a very elastic general 

 formula for the sort of causal connection 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' plants, the permanent growth movements of tropistic curvatures, or 

 the complex changes of plant-form and development that result from present and 

 past variations of external 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 implied about the mechanism connecting the cause and the effect, 

 and nothing even about the relative magnitudes of these, but all this is lelt for 

 special research on the case under consideration. The one natural chain of cause 

 and eifect 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 by the turning of a tap. 



So elastic a conception may be easily stretched to fit almost any sequence of 

 phenomena with the apparent closeness that argues a bespoken garment. We 

 must therefore be critically on our guard against cases of such sartorial illusion. 



The Principles of Chemical Mechanics. 



That my consideration of particular cases may be intelligible it seems 

 necessary that I devote a few minutes to outlining the four quantitative 

 mechanical principles which govern e\ ery single chemical reaction, though much 

 that I have to say has been drawn from elementary books on physical chemistry. 



These four principles are concerned with (1) the nature of the reaction in 

 question; (2) the amount of reacting substances that happen to be present; 

 (3) the temperature at which the reaction is taking place ; and (4) the influence 

 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. 



(1) The first principle that we have to consider is that which declares that 

 no chemical reaction is really instantaneous, 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 reaction-velocity which distinguishes 

 it from other reactions. This is expressed by giving to each particular reaction a 

 numerical velocity-coefficient which is low or high proportionally as the reaction 



slow or quick. 



(2) This coefficient only expresses the actual experimental velocity when 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 A of unit concentration is undergoing change, then 

 to keep this reaction going at its present rate fresh amounts of a must be added 

 continually 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, 

 i.e., of the specific velocity of this reaction. 



