INSECTICIDES, FUNGICIDES, ETC. 157 



paraffin in a soap solution. A concentration slightly less than this 

 (say, 67 per cent.) is the best to aim at in making a stock emulsion, 

 to be diluted with water when required for spraying. One volume 

 of a i or 2 per cent, solution of soft soap should have two 

 volumes of paraffin added to it in small quantities at a time, 

 churning the mixture between each addition. It is preferable, but 

 by no means essential, that the soap solution should be warm. 

 Emulsincation appears to occur suddenly, and its occurrence is 

 recognisable by the increased difficulty in working the syringe. 

 A stock emulsion of the above strength will require dilution with 

 from 10 to 50 times its volume of water before use. 



Even from a 74 per cent, emulsion, some watery liquid will 

 generally separate after a time, for the globules constituting the 

 cream are lighter than the water, and press upwards, becoming 

 somewhat distorted from their spherical shape in order to pack 

 closer together. Such closer packing may be carried much further 

 by churning the cream with more paraffin ; the cream becomes 

 thicker on each addition, till it is almost solid, like a blancmange, 

 and eventually forms a translucent jelly with a bluish tinge. 

 An emulsion of this sort containing as much as 99 per cent, of 

 paraffin has been obtained. The particles of paraffin present in 

 it can then no longer be spherical, but dodecahedra, closely fitting 

 against each other, and separated only by the thinnest film of 

 soap solution ; under the microscope its structure is no longer 

 visible. On exposing it to dry air, the film dries up, and the 

 whole is transformed into a mass of liquid paraffin; whereas 

 on exposing it to moist air, or on putting it into water, it 

 becomes opaque, the particles of paraffin resuming their globular 

 form. 



The reason why a soap solution, and not water, will allow of 

 paraffin becoming emulsified in it, had always been assumed 

 to be that the soap alters what is known as the surface tension 

 between the water and the oil ; but the Woburn results showed 

 that this explanation is insufficient, for scores of instances were 

 found where emulsification occurred without any alteration in 

 surface tension, and the results with soap-solution itself disprove 

 such a theory, for the alteration in surface tension progresses 

 regularly as the amount of soap is increased, whereas the power 

 of emulsifying does not; it disappears entirely when the soap 

 present exceeds a certain limit. Thus, to get an emulsion 

 containing 75 per cent, of paraffin, the soap solution must not 

 contain more than r8 per cent, of soap; to get one with 40 per 

 cent, of paraffin, the limit of strength of the soap solution is 



