G. H. F. Nuttall 
485 
Tlie insecticides tested by Labbe and Wahl were more or less volatile 
and diffusible. The reader is referred to the original for further details, 
but it may be stated here that the whole subject requires scientific 
investigation. 
Knaffl-Lenz (1915, p. 708) believes that acid vapours are the most 
potent because they neutralize the alkaline body-fluids of the insect 
and prevent the giving off of CO., as observed in poisoning by acids; 
such vapours are best used in practice because the insects do not recover 
from their effects. Neutral and alkaline vapours, on the other hand, 
do not enter into chemical combination in the louse’s body; they are 
dissolved and if their action is not prolonged their effects are transitory. 
The acid vapours commonly used are sulphur dioxide and acetic acid, 
and formic acid is suggested as worthy of trial. 
Many experiments conducted in vitro, are found worthless when the 
results are applied in practice. This may be due to several causes. 
Many authors, as some of the following protocols show, fail to state the 
conditions under which they conducted their experiments, two matters 
of fundamental importance, namely the vapour concentration and the 
temperature frequently receiving no mention. The essential oils mostly 
stand condemned on the grounds: (a) that their effects are transitory 
because they evaporate too rapidly; ( b ) that in practice as a rule they are 
too expensive; (c) their smell is frequently objected to and some persons 
exhibit idiosyncrasies with regard to their effects; ( d) some of them, 
whilst they rapidly stupefy the active stages, fail to exert an appreciable 
effect on the nits. According to Sikora (vm. 1915, p. 529) a small amount 
of the vapour of several essential oils inhibits the development of nits 
in vitro and the author regards the negative results in killing nits by 
such vapours as due to the too short exposure. There is however a 
general consensus of opinion to-day that the vapours of essential oils 
only suffice in practice to stupefy the insects temporarily without 
affecting the nits, consequently they are regarded merely as palliatives. 
From a practical standpoint the exact mode of action of an insecticide 
is immaterial provided it exerts the effect desired. At this juncture, 
therefore, we need not dwell further on the subject. 
Choice of Insecticides and Mode of Application. 
The choice and manner of applying insecticides in practice depend 
entirely upon the conditions which have to be met, and whether they 
are to serve merely as palliatives or not. Insecticides or mixtures 
