INSECTICIDES, FUNGICIDES, ETC. 159 



an emulsion made with soap generally becomes de-emulsified. 

 De-emulsification, once it begins, progresses rapidly. 



Spontaneous de-emulsification does not occur with solid emul- 

 sifiers, such as the basic sulphates, and emulsions made with 

 them may be kept for years without any visible change. 

 Another great advantage which they possess is that a caustic 

 alkali may be added to them, so as to increase their detergent 

 properties, without causing de-emulsification ; with a soap emul- 

 sion the addition of caustic soda to soft (potash) soap converts 

 this into the nearly insoluble, flocculent, soda soap, and destroys 

 the emulsion ; whilst the addition of the more expensive potash 

 causes gelatinisation unless the emulsion is very dilute. 



It was, indeed, an endeavour to obtain an alkaline emulsion 

 which led to the discovery of the power of finely divided solids 

 to act as emulsifiers. A mixture of lime with paraffin had been 

 used in the United States, under the name of limoid, for spraying 

 purposes ; but an examination of such a mixture showed that it 

 was not really a true emulsion; the globules of oil in it were 

 comparatively large, and irregular in shape, appearing to be 

 only entangled in the particles of lime : when water was added 

 to it, some of the lime settled to the bottom, carrying with it 

 a little of the oil, whilst the bulk of the oil rose to the surface, 

 carrying with it some of the lime. Moreover, the mixture, 

 unlike a true emulsion, made the containing vessel oiry. Such 

 mixtures have been termed quasi-emulsions, and many fine 

 powders besides lime may be used to make them. The addition 

 of caustic soda to a soap emulsion converts it into such a quasi- 

 emulsion, the texture of which varies considerably with the 

 nature of the soap used. In all quasi-emulsions there are some 

 particles present sufficiently small to act as true emulsifiers, 

 hence every gradation is possible between a true and a quasi- 

 emulsion. It was found, in the case of the lime mixture, that 

 the addition of some copper or iron sulphate resulted in the 

 formation of a very different product, namely, a very perfect 

 true emulsion, which does not render the vessel oily, and which 

 is not separated at all into its components on the addition 

 of water, but which rises or sinks as a whole, according to 

 the proportions and densities of the substances forming it. 

 It was the basic sulphates, formed by the action of the lime on 

 the sulphates, which acted as emulsifiers of the paraffin. To 

 obtain the most perfect emulsions, no excess of lime should 

 be present; clear lime water should be added to the solutions 

 of the sulphates, and on churning paraffin with the resulting 



