142 Accessor// Wetting Substances : Paraffin Emulsions 



In this connection paraffin emulsions were tried. These emulsions 

 have been often recommended by many writers as insecticides and 

 there is no doubt that they are of value. They have, however, a 

 reputation of being dangerous to use owing to their Uability to scorch 

 foliage. There are, the author believes, very definite reasons for this 

 liabihty but by suitable means it may be got rid of. 



An emulsion may be stable or unstable, its state depending on its 

 constitution. If stable, every small paraffin globule is surrounded by 

 a film of soap and the film is sufficiently strong to keep one globule 

 from touching and coalescing with its neighbour. If very httle soap 

 is originally present, or if where present in sufficient quantities it is 

 withdrawn by chemical means, there comes a point where the film is 

 no longer strong enough to prevent paraffin globules from touching 

 and fusing with their neighbours. When this happens throughout the 

 emulsion, paraffin appears in quantity on the surface and the emulsion 

 is said to be unstable. 



Pickering! has pointed out the great diversity of formulae for paraffin 

 emulsions that have been recommended. In a table given by him the 

 proportion of paraffin to soap in one formula is as great as 100 : 1-2, 

 in others as low as 100 : 240, while there are all kinds of intermediates. 

 This enormous range in quantities suggests that some of these formulae 

 have been advocated in a somewhat haphazard spirit, since if formulae 

 at one end of the series were successful it does not seem necessary to 

 make use of others which contain such divergent quantities. 



Of the 24 formulae quoted 15 contained paraffin in proportion to 

 soap in ratios varying from 100 : 23 up to 100 : 1-2. That is to say 

 in those mixtures of these fifteen where the paraffin bore the lowest 

 ratio to the soap, it was present to the extent of four times the amount 

 of soap and in those which bore the highest ratio, to as much as eighty- 

 three times. It was therefore at first sight probable that the liability 

 to scorching was caused by the high content of paraffin. Experiments 

 were therefore started to see how little paraffin need be used in order 

 to be letlial and at the same time to avoid scorching. It was at once 

 found that with the Bristol water which has a hardness of about 12° 

 it was impossible to obtain any kind of stable emulsion with even small 

 quantities of paraffin unless at least | % of soft soap were used. The 

 paraffin simply floated on the surface of the liquid and when sprayed 

 on to foliage naturally produced scorching. 



In this connection it is interesting to notice that out of 23 formulae 

 ^ Qth Report, Woburn Exper. Fruit Farm, 1906. 



