88 PRINCIPLES OF GENERAL PHYSIOLOGY 



greater will be the buoyant effect of the bombardment on the part of the water 

 molecules. 



The presence of an electric charge will also tend to prevent aggregation, on 

 account of mutual repulsion. If by any means a number of the particles are given 

 opposite charges to the remainder, aggregation will naturally be brought about by 

 mutual attraction. This question will be discussed below. That the electric 

 charge is not the sole cause of permanent suspension is shown by the fact that it 

 can be reduced to zero, without affecting the stability, as in the experiment of 

 Svedberg, given on page 84 above, where the Brownian movement was unaffected. 



The opposing action of mechanical surface tension and electric charge has 

 already been indicated. Lewis (1909, 3) shows how, with a given electrical charge, 

 at a certain definite radius of the particle, the surface energy will be at a minimum, 

 and therefore the stability at a maximum. It will be remembered that the 

 surface tension is tangential and the electric force radial, so that it is only the 

 radial component of the former which is opposing the electric force. This latter, 

 however, acts inversely as the fourth power of the diameter, while the former acts 

 inversely as the simple diameter. 



The viscosity of the external phase should also be referred to. Increase of 

 internal friction of the medium of suspension will increase the time taken for 

 particles to fall under the action of gravity. 



THE COLOUR OF SOME HYDROSOLS 



Interesting evidence of the gradual transition from molecules to colloidal 

 particles is afforded by the work of Svedberg (1909, 2) on the colour of gold 

 hydrosols. With increasing dispersion, that is, more minute subdivision, the 

 colour of the colloidal solution of gold approximates more and more to that of a 

 gold salt in true solution, or the colour of the gold ion, supposing the anion to be 

 colourless. The absorption in the spectrum shifts more and more towards the 

 ultra-violet, where gold chloride possesses a characteristic absorption. Wohler 

 and Spengel (1910) have shown also that coarsely colloidal platinum is of a more 

 or less violet colour, which becomes more and more like the orange colour of 

 platinum salts as the dispersion is increased. Wo. Ostwald (1911) shows that the 

 maximum of absorption, as a general rule, gradually passes to the shorter wave 

 lengths as the particles become smaller, so that the colour of the solution, that is, 

 the colour of the light transmitted, changes from blue or green to red and yellow. 

 For further details, the reader is referred to the interesting article by the last 

 named author. 



The relationship between the dimensions of the particles and the wave length 

 of the light absorbed obviously suggests effects of resonance, or simple relationship 

 between the rate of vibration of the particle and that of the light absorbed. 



This phenomenon of resonance enables a considerable amount of energy to be accumulated 

 from a series of periodic impulses, each of a very minute energj", and deserves a little con- 

 sideration. Suppose a pendulum with a rate of vibration of one second, reckoned as the time 

 elapsing between the passage through any position, and the next passage in the amf. direction. 

 If we start with such a pendulum at rest, and give it a very slight push in the plane of its 

 vibration, and repeat this at intervals of one second, it is possible to get up a considerable 

 amplitude of vibration ; each impulse adds its effect to that of the previous ones. Unless the 

 interval between the periodic impulses is a multiple of the time of vibration of the pendulum, 

 only a very small amplitude, if any at all, will be obtained, since it will only occasionally 

 happen that the impulse is delivered in the same direction in which the pendulum is moving ; 

 all other impulses will retard the movement, energy from the pendulum being given back to 

 the body producing the periodic impulses. 



This resonance process plays a large part in decomposition by light and, if 

 we remember the rates of vibration of light and of molecules, we realise the 

 possibility of considerable energy changes in comparatively short times. The 

 rate of vibration of the light of the D line of sodium is, in fact, about 5 x 10 14 per 

 second. 



Resonance .also comes into play in the production of powerful high frequency 

 electrical discharges, as used in electro-therapeutics, and in the action of the 

 auditory apparatus, according to the theory of Helmholtz. 



