54 COMPARATIVE PHYSIOLOGY 



total absence of oxygen brings about cessation of luminescence 

 in the moistened material, a very low tension of oxygen (3 mm. 

 in the case of Cypridina) suffices to maintain the emission of 

 light. Measurements of the electrical conductivity of lumi- 

 nescent solutions prepared in this way by E. N. Harvey show 

 no progressive increase in H-ion concentration indicative of 

 CO2 production. 



Bioluminescence, like the secretion of digestive fluids, 

 affords an instance of the production of the substances known 

 as enzymes for specific ends. And a few words may here be 

 inserted with reference to the conception of an enzyme. 

 According to their velocities, molecular reactions may be 

 divided into two categories, those involved in the familiar 

 methods of volumetric analysis, where the combination of 

 reacting substances is practically instantaneous — ^.^.precipita- 

 tion of barium salts in the presence of SO4 ions or the union 

 of hydrochloric acid gas with ammonia — and a large class of 

 organic reactions, such as the saponification of esters or inversion 

 of cane sugar, for which an appreciable interval must be allowed 

 to elapse if a condition of equilibrium is to be attained. 

 Reactions in the living body are for the m^ost part of the latter 

 type. It is a familiar fact that many reactions which ordinarily 

 proceed at an immeasurably slow rate take place with great 

 rapidity in the presence of substances known as catalysts, 

 which though influencing the velocity of the reaction do not 

 enter into the composition of the end-products or in general 

 shift the point of equilibrium for a given set of conditions. 

 The facility with which the organism is able to disintegrate 

 highly stable compounds is due to the agency of a special type 

 of catalysts known as enzymes, characterised especially by their 

 extrem^e instability (which is illustrated by the fact that 

 practically all enzymes are rapidly destroyed at temperatures 

 well below the boiling-point of water), and by the extremely 

 minute quantities in which they act. Thus the preparation 

 known as rennet, of which the active constituent represents 

 a very small fraction of the total bulk, is able to clot 400,000 

 times its own weight of the milk protein caseinogen. 



Of the general catalytic properties of enzymes, the most 



