and the Action of the Bile in Digestion. 303 



for emulsion-development, not in the soap dissolved in the 

 liquid, but in that which forms more or less rapidly on the 

 surface of the oil, where, in fact, the fatty acids contained in 

 the oil come into contact with the alkaline liquid. He then 

 proceeds to observe that when the soap that has been formed 

 under the given conditions is soluble in the surrounding liquid, 

 it is by diffusion carried off radially from the point where it is 

 developed, while inside the globule the equilibrium of solu- 

 bility is maintained by diffusion of the fatty acid outwards 

 towards the periphery. If the soap be so rapidly formed and 

 removed that the fatty acid by diffusion towards the periphery 

 cannot make up the deficiency, the outer edge of the globule 

 will alter its shape, and there will be thrown off smaller fatty 

 particles which are not enclosed in a soapy membrane. But 

 when the rapidity with which the soap is formed reaches a 

 certain point, the latter will not be dissolved by the surround- 

 ing liquid, and the oil-globule will be enveloped by a soapy 

 membrane. The development and displacement of the soapy 

 matter will give rise to a change of volume in the liquid ; and 

 this in its turn will cause the breaking-away of small fatty 

 particles, each of which is enclosed in a film of soap. Should 

 there be from any cause an irregularity in the course of the 

 formation of the soapy membranes, it will occasion Amoeba- 

 like movements, the protuberances being extended at the points 

 where the membrane takes a longer time to thicken. 



To these theoretical views, however, I am unable to assent, 

 because it appears to me to be highly improbable that mere 

 diffusion currents can produce movements so energetic as those 

 that are observed in the production of emulsions. In the fol- 

 lowing pages I shall endeavour to prove that the formation of 

 an emulsion depends essentially on the existence of thin scales 

 of soap solution dispersed over the common surface of the oil 

 and the liquid, and also that the so-called Amoeba-like move- 

 ments depend on the same cause. 



2. The Bounding Surfaces of Liquids in Contact with Air 

 and with Water, 



The surface of every liquid, whether the same be bounded 

 by air or by another liquid , has a tendency to become as 

 small as possible, or, as it is commonly termed, has a certain 

 tension. The magnitude of this tension, which may be likened 

 to that of a cloth, or to that of the envelope of a caoutchouc 

 balloon, or of an inflated pig's bladder, is measured by the 

 force (in milligrammes) exerted on a strip of the surface one 

 millimetre broad. The tension of the surface of a liquid 

 bounded by air is at the same time a measure of the cohesion 



2B2 



