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him to the American physicist, Millikan (2). The phenomenon 

 investigated by Perrin, a phenomenon which at first sight 

 would seem to have very little to do with the existence of 

 molecules, was the well-known Brownian movement of small 

 particles suspended in a liquid medium. If, for example, we 

 examine by means of a high-power microscope a liquid in 

 which very fine particles are suspended, such as gamboge in 

 water, or the still finer particles of colloidal solutions, which 

 can be rendered visible (indirectly) by means of the ultra- 

 microscope, we find that these particles are in a state of rapid 

 and irregular motion. Early investigations of Ramsay, and 

 still more those of Gouy (3) had served to demonstrate that 

 the most likely cause of this movement was the bombardment 

 of the particles by the molecules of the surrounding medium. 

 Perrin succeeded in putting this conclusion to a quantitative 

 test. He showed, for example, that the suspended particles 

 distributed themselves in such a manner that their concentra- 

 tion at the bottom of a cylinder was considerably greater than 

 that at the top, and that, in fact, the concentration varied 

 quite regularly according to a simple law. Having obtained 

 the necessary data in a fairly exact form, he proceeded to apply 

 certain theoretical considerations which necessarily follow 

 from the assumption that the Brownian movement is due to 

 molecular bombardment and is at the same time governed by 

 the principles of the classical statistical mechanics, i.e. the 

 principle of equipartition of energy. In this way, Perrin 

 succeeded in establishing a relation between the concentration 

 gradient of the gamboge particles and the quantity N, a quan- 

 tity which stands for the actual number of molecules in one 

 grammolecule of a gas, to which Perrin gave the name, the 

 Avogadro Constant. The value thus obtained for N agreed 

 so well with certain approximate determinations which had 

 previously been made that there could be no reasonable doubt 

 but that the Brownian movement was actually due directly 

 to bombardment by molecules. Hence, although we have 

 not yet succeeded in seeing a molecule, we have actually 

 before our eyes an irregular motion analogous to molecular 

 motion itself and directly caused by the movement of mole- 

 cules. Later on, Millikan improved the technique of such 

 experiments by following the movement of a tiny droplet of 

 oil suspended in air, and has in this way arrived at a very 



