726 PRINCIPLES OF GENERAL PHYSIOLOGY 



whence we obtain 57,625,000 for the " molecular weight" of the muscle cell, truly 

 a " giant " amongst molecules. The participation in a chemical reaction of such a 

 molecule is difficult to realise, as also how a comparatively small molecule attached 

 at one point should influence the whole of the giant. Naturally, the molecule 

 with which combination takes place may be only one of those present in the cell, 

 but then we have to give up the theory of giant molecules. 



The only remaining point of general application to which I would call attention 

 is the nature of " specificity " in relation to these and similar systems. In con- 

 nection with enzymes, I have discussed the question at some length in an appendix 

 to my monograph on " Enzyme Action," and shown that the preferential action of 

 an enzyme on a particular substrate is, apparently, only a matter of rate, and if 

 so, that the " lock and key " illustration is not quite appropriate. If a key does 

 not fit it will never open a lock, however long a time be given for it to do so. In 

 fact, it seems to me that such a point of view, met with also in the form of a 

 " moulding to templates," is not applicable to reactions in the organism, if to any 

 chemical reaction at all. The kinetic view of velocities of reaction is more in 

 accordance with facts. 



In a previous page (page 60), in speaking of " specific " adsorption, it was 

 pointed out that, in this process, there are innumerable possibilities of interaction 

 of the various forces acting at surface boundaries, without the necessity of 

 calling in the provisions of chemical groups which are to be supposed capable of 

 combination with groups on the substance adsorbed, and with these particular 

 groups only. Attention may also be directed to the work of Barger and W. W. 

 Starling (1915) on the adsorption of iodine by certain organic compounds. This 

 shows itself to be closely related to the chemical composition of these substances 

 and therefore to that of their surfaces. But this relationship does not result in 

 chemical combination nor in abolition of the nature of the process as an adsorption. 

 It would appear that those properties of the surface, such as electric charge and 

 so on, which control the degree of adsorption, are dependent on the chemical 

 nature of the surface. This dependence need cause us no surprise, since the 

 physical properties of a substance, inclusive of surface energy, are so closely 

 related to its chemical composition. 



When we have to deal with colloidal solutions, the considerations brought 

 forward by Wolfgang Ostwald (1912) are to the point. We saw (page 107) that, 

 when two colloids have charges of opposite electrical sign, it is usual to find that 

 they mutually precipitate one another. Also that a colloidal solution is 

 precipitated by ions of the opposite electrical sign to themselves. Now, Michaelis 

 and Davidsohn (1912) found that the precipitation phenomena in cases of some 

 " precipitins " and " agglutinins " are nearly independent of the hydrogen ion 

 concentration in the solution, and, therefore, of the electrical charge of the 

 particles. The conclusion drawn is that there is a specific chemical affinity 

 between the substances concerned. But, as Wolfgang Ostwald points out, all 

 that the experiments show is that processes of purely electrical neutralisation 

 are insufficient for the purpose. But it is not to be supposed that electrical 

 charges are the only properties of surfaces that concern colloidal systems. We 

 have only to remember the " coagulation " by neutral salts, by non-electrolytes, 

 by the results of " mechanical " adsorption on surfaces, and so on. Moreover, even 

 when electrical charges are present, the precipitation is not always determined 

 by them. For example, tannin precipitates gelatine better in the presence of acid ; 

 gold sols are not necessarily precipitated when deprived of charge. The chief 

 variable, surface tension, is altered, not only by electrical charge, but also by 

 chemical composition, temperature, degree of dispersion, degree of solvation, 

 and so on. The amount of electrolyte required to precipitate sulphur sols, as 

 mentioned above (page 94), depends on the size of the particles. 



It is not permissible, in fact, to refer all hitherto unexplained phenomena 

 to chemical relations, as is customary. Substances are frequently assumed to 

 be chemically different because they have some different physical property, 

 although no chemical difference can be detected. Tannin, for example, has 

 a higher optical activity in a higher dispersed condition than in a coarsely 



