METHODS 



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which Dujardin discovered intuitively. Nowadays we 

 say protoplasms are colloids which amounts to asserting 

 that every substance, while living, is in the colloid state. 

 The interest of this simple proposition is beyond measure ; 

 it is the keystone of all biology. 



Let us not forget, however, that one of the first conquests 

 of comparative physiology was to establish the existence 

 of many chemical phenomena among vital phenomena. 

 Now chemistry deals with atomic or molecular dimensions ; 

 the colloid state, on the contrary, relates to activities of 

 a dimension far superior to that of molecular reactions ; 

 and so we find, for the second time, this remarkable charac- 

 ter in biological phenomena, that they take place, so to 

 speak, at one and the same time, along two different scales 

 of magnitude. In fact, as we have already remarked, 

 living substances which are sensitive to sound are also 

 sensitive to light ; in like manner, we now see that vital 

 reactions are at one and the same time of chemical and col- 

 loid order ; and so by an hypothesis, which may seem bold, 

 but which opens vast horizons before us, we may suppose 

 that, if light acts on the chemical reactions of life, sound 

 acts on its colloidal manifestations. 



A most important conclusion which already seems to 

 result from this very recent study of colloids a conclusion 

 which will become more and more clear in the following 

 pages is that, in many cases, the chemical reactions pro- 

 duced between the particles in suspension and the liquid 

 solvent help to modify the nature of the colloids considered 

 as colloids ; in other words, they have an influence on the 

 colloidal state special to the particular colloid. Reciprocally, 

 if direct actions modify the colloidal state, there may be a 

 result in chemical variations, molecular reactions, between 

 the suspended particles and the solvent. 



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