PHYSICAL METHODS IN PHYSIOLOGY 83 



surrounding the living cell. The permeability of colloidal 

 membranes and the effects of salts and electric forces upon 

 them ; the electric charges on colloidal particles, and the 

 aggregation, precipitation, or dispersion of such particles under 

 the influence of physical or chemical agencies ; the phenomena 

 of surface tension, of adsorption, of katalysis in colloidal 

 systems — all these are of surpassing interest to the physiologist 

 in his study of the fundamental nature of the physical accom- 

 paniments of vital activity, but of a complexity still unrelieved 

 by any really satisfactory generalisation, enabling him to 

 understand their behaviour. Why is a precise balance of 

 various salts so essential to the living creature ? What pro- 

 cesses, chemical or physical, underlie the phenomena of im- 

 munity, of haemolysis, of agglutination ? What is the nature 

 of the electric change which is the only known accompaniment 

 of the transmitted nervous impulse ? What — even in the most 

 general way — is the mechanism by which chlorophyll stores up 

 the energy of sunlight ? Why do two bodies, almost identical 

 in chemical and physical properties, have so completely different 

 effects upon the living cell ? All such questions — questions of 

 the most general interest and of the most fundamental appli- 

 cation — are at present unanswerable. They will be answered 

 only by a more searching analysis of the geometrical, as well as 

 of the chemical and physical, properties of the living cell, and 

 of the colloidal and chemical bodies occurring in its structure. 

 Colloidal chemistry is an attempt to deal with the individual, 

 as well as with the collective and average properties of par- 

 ticles of matter, and of the molecules, atoms, and electrons 

 which compose them ; such individual treatment is an essential 

 step in the study of the mechanism of the living cell. 



Up to recent times it was customary to adopt a philosophical 

 and non-committal attitude towards the molecule, the atom, 

 and the formulae of stereo-chemistry. Although one might 

 describe the behaviour of the molecule by means of a structural 

 formula, this formula might be nothing more than a working 

 hypothesis very far removed from the real facts. The evidence 

 for the structural formula was indirect — it involved no direct 

 use of spatial measurements. To-day the position has changed, 

 and convincing evidence for the real spatial existence of the 

 molecule, and of the atom and its parts, has been produced. 

 The laws of aggregates, such as the Gas Laws, the Second Law 

 of Thermodynamics or the chemical Laws of Mass Action, might 

 enable one to evade the mechanism, and to obtain results true 

 on the average and in the mass ; but the mechanism was still 

 there — in space as well as in time — and such laws had no applica- 

 tion to it. If, then, the atom, the molecule, the colloidal particle, 

 have a spatial existence, we may expect their properties — their 



