264 LECTURES ON IMMUNITY 



of clay. Shall we suppose now that the coagulation of the 

 casein changes its value rapidly in the same moment that 

 we are able to observe its progress ? From this point of 

 view the phenomenon manifests itself to our eyes as a 

 steadily increasing molecular condensation. It displays the 

 behaviour of clay particles which aggregate and subside. 

 . . . We are therefore led to suppose that this regular con- 

 densation, which causes the coagulation as far as we are 

 able to observe it, begins already before the microscope 

 can detect it. But however well grounded this induction 

 may seem to us, it would remain unfounded if we could not 

 control it by experiment." 



This experimental proof Duclaux finds in the reaction 

 of Tyndall. Ultramicroscopical powders suspended in an 

 absolutely clear fluid reveal their presence by the produc- 

 tion of a blue colour on illumination by a ray of light, and 

 this blue reflected light is polarised. This phenomenon 

 is displayed, for instance, by a suspension of fine particles 

 of mastic, prepared by adding a few drops of an alcoholic 

 solution of mastic to water. If we add greater quantities 

 of the solution, the light reflected becomes more .pale and 

 white; and at a certain point it is possible by means of the 

 microscope to detect in it small particles that show the 

 Brownian movement. At a still higher concentration of 

 the solution a real precipitate is formed, which is easily 

 observed with the naked eye. 



To this argument may be added still another fact. Such 

 submicroscopic particles may be observed by means of 

 the ultramicroscope of Siedentopf. Solutions of proteins 

 generally reveal the presence of such submicroscopic 

 granules, and this observation has been interpreted in 



