SOME ASPECTS OF SPRAYING AGAINST PESTS. 
219 
to overcome the second difficulty by the introduction of paraffin to 
form an emulsion with the soap. Suitable emulsions have greatly 
enhanced wetting powers, and by that means it is possible to wet with 
ease even the most resistant surfaces. 
It is undoubtedly true that paraffin emulsions have earned for 
themselves a bad name owing to the tendency of some of them to 
produce burning of the foliage. When one examines some of the 
many formulae that have been advocated, one is struck by two facts : 
firstly, the great variation in the ratio of soap to paraffin, and secondly, 
the relatively high proportion of paraffin used in many of them. The 
great amounts of paraffin and the small amounts of soap used where 
the paraffin content is low constitute one of the reasons for liability 
to burn. Another reason may be found in the imperfect making of 
these emulsions. The errors in making may be best understood after 
a consideration of the structure of emulsions. 
A perfect emulsion of paraffin in soap solution consists of numerous 
minute drops of paraffin, each surrounded by a film of soap. It is 
this film of soap which prevents any one paraffin droplet from touching 
and coalescing with its neighbour. As long as the soap solution is 
strong enough to prevent this tendency the paraffin remains in proper 
emulsion, but as soon as it becomes so diluted that one paraffin drop 
can join its neighbour de-emulsifi cation sets in and free paraffin rises 
to the surface. It is therefore clear that if the soap be not present in 
sufficient quantity the emulsion is unstable and therefore dangerous to 
use. This condition may arise either through insufficient soap being 
present in the first instance or through dilution of an otherwise stable 
emulsion. It is also clear from what has been said that the efficiency 
and freedom from burning will depend on the smallness of the paraffin 
drops. It is important, therefore, to see to it that in the first instance 
the paraffin shall be well broken up. This is best effected on 
a small scale by violently syringing the paraffin into a hot soap 
solution, using an ordinary garden syringe with a rose attached 
for the purpose. On the large scale an emulsion may be best obtained 
by placing the mixture of paraffin and soft soap solution in the con- 
tainer of a spraying machine and pumping it with violence through 
the pump back into the container through a narrow jet. If made 
by simply stirring or agitating paraffin with soap solution, an imperfect 
emulsion is obtained and the resulting mixture is almost sure to scorch. 
It is unnecessary to go into details here, as it is hoped shortly to 
publish a paper on the subject. It will be sufficient to summarize 
the results. With water of moderate hardness, such as is represented 
by the figure 12 1 of the hardness scale, it is possible to divide the 
various emulsions into three classes. These are : 
(1) The unstable and dangerous. 
(2) The risky. 
(3) The safe. 
The first class, or unstable, consists of those which contain so little 
soap that free paraffin soon appears on the surface of the liquid. An 
