THE COLLOIDAL STATE 87 



These figures are also instructive as showing what complexity results from the action of 

 apparently simple and uniform forces. The mean kinetic energy of each molecule is the 

 same as that of other molecules, and the forces to which it is exposed might be imagined to 

 be symmetrically distributed in the body of the liquid, and yet we obtain this apparently 

 "chaotic" variety of movement. It is unnecessary to remark that it is not really in any 

 way " chaotic," the impression it gives us is merely due to our inadequate methods of 

 observation. By this second method a value of N of 71 '5 x 10 22 was obtained. 



The third method depends on the fact that, when the dimensions of the 

 particles are sufficiently large, many of the impacts of the water molecules 

 will be directed more or lass tangentially, and so cause rotation of the particles, 

 which can be observed when these contain some distinguishing mark, as an 

 inclusion in course of their formation. A formula, also due to Einstein, gives 

 the possibility of another determination of N, which comes out as 65 x 10 2 ' 2 . 



If we compare these various values with the latest and most accurate measure- 

 ment by Millikan (1910) (see Perrin, p. 84), by the method of electric charge 

 on gas ions, which gives 62 x 10 with an uncertainty of only 2 per cent., we 

 must be struck by the very close agreement, and have no hesitation in admitting 

 the truth of the view that Brownian movement is the same thing as the 

 molecular movement of the kinetic theory. Perrin's latest results (1911, 

 pp. 1-2), indeed, give values still closer to the number found by Millikan. 



Experiments were also made by the second method with much larger particles 

 in 27 per cent, solution of urea in order to keep them in suspension. These 

 gave a value for N of 78 x 10 22 . Considering the small number of observa- 

 tions made, the agreement must be regarded as satisfactory. Since the founda- 

 tion of Einstein's theorem is the assumption of equal partition of kinetic energy, 

 and the experiments showed that particles differing in diameter 60,000 times 

 gave the same value of N, they must be looked upon as the most weighty 

 confirmation of the hypothesis of equal partition of kinetic energy. 



It should be remembered that Ramsay (1891) advocated the view that Brownian movement 

 is due to the impacts of molecules of the liquid against the particles, and that Ramsay and 

 Senter (British Association Reports, 1901) concluded from the fact that the density of colloidal 

 solutions of arsenious sulphide is the same, whether measured by the hydrometer or by 

 weighing, that the particles of the colloid hit against the hydrometer to float it with the 

 same energy as the molecules of the water do. 



It is impossible to avoid some satisfaction that further evidence is given by 

 Perrin's experiments, that we are not compelled to be content with equations 

 derived from energetics, since the visible particles of these experiments behave 

 precisely like the supposed molecules of the atomic theory. The chemist may also 

 regard his structural formulae with more satisfaction of their approximate 

 resemblance to actual fact. Incidentally, van't Hoff s theory of solutions receives 

 confirmation. 



In connection with the illustration of the kinetic theory afforded by the Brownian move- 

 ments, as pointed out above, attention may be called to the fact that theories dealing with 

 the movement of molecules, such as the kinetic theory of gases, are essentially statistical, that 

 is, they are not concerned with the actual energy possessed by an individual molecule at a 

 given instant of time, but with the average of a very large number. If the energy of a single 

 molecule at a given moment of time could be measured, it might be found to be a very long 

 way off from the mean. 



This consideration is probably that which lies at the basis of the possibility, to which 

 Donnan has called attention, that a living organism might appear to evade the second law of 

 energetics. If we look upon an individual organism as a molecule in respect to the world 

 of similar organisms, it does not seem, prima facie, altogether impossible that its activities 

 might so far differ from the mean as to contravene the laws deduced from the general mass. 

 But, in point of fact, we do not meet with deviations of this kind. We know, for example, 

 that if we stimulate the vagus nerve, the heart will certainly stop, except some counteracting 

 agency, such as atropine, is present, which we can lay our finyer upon and allow for, in 

 due order. 



OTHER CONDITIONS OF STABILITY 



Although the Brownian movement is the chief cause of the permanency of the 

 colloidal state, there are some other conditions which play a part. The density 

 of the medium in which the particles are suspended will clearly have an effect, 

 the greater the density, the less the effective weight of the particles, hence the 



