W. F. Cooper and H. E. Laws 
199 
cause the solution to run between the spherical and flat surfaces so as 
to wet the skin of the host thoroughly (Fig. 2, B). This is precisely the 
object of adding an emulsion to the aqueous solution of sodium arsenite; 
the surface tension 1 is materially reduced thereby, and the arsenic is 
brought into intimate contact with the tick’s proboscis and the skin of 
the host immediately adjacent and is thus enabled to exert its greatest 
effect, for as the tick then sucks up its food from the host, it is almost 
certain to imbibe a lethal dose of arsenic. If, on the other hand, an 
air-gap is formed round about the tick’s proboscis, the arsenic is some 
distance removed from the point of insertion and the amount sucked 
up would be comparatively small. 
Though Watkins-Pitchford showed the great value of an emulsion, 
he does not appear to have made any observation on the physical 
phenomena involved, though an exa m ination of his results absolutely 
confirms the foregoing explanation. 
Does the Tick take up Arsenic from, the Blood or from the 
Skin of the Dipped Host ? 
For the furtherance of our inquiry, it seemed to us that the question 
as to whether the arsenic, imbibed by a tick while feeding on a dipped 
host, was derived from the blood or the skin, must be settled definitely. 
With this object in view, we carried out a series of experiments in which 
arsenic was administered, per os, up to the maximum possible dose, yet 
no ticks were killed (see Appendix III). 
It may be argued that with internal dosing, no great amount of 
arsenic reaches the circulating blood owing to the rapid excretion from 
the bowel, due to purgation, arrest in the liver, etc. In order to overcome 
this difficulty and to ensure the presence of an active quantity of arsenic 
in the circulating blood we then administered the doses in the form of 
o 
subcutaneous injections, again without positive result (see Appendix IV). 
Finally, experiments were carried out, in which a solution of arsenic, 
much stronger than could be used for general application in the form 
of a dip or spray, was distributed over a limited area of the skin. In 
these experiments the ticks on the treated areas were killed, but the 
1 The term ‘Surface Tension’ is used here in a general sense only. It is now realised 
that the wetting power of a solution is dependent upon other factors in addition to the 
surface tension of the wetting fluid. A paper, by one of the present authors (W. F. C.) 
in collaboration with another, dealing with the Theory of Wetting and the Determination 
of Wetting Power, is in course of preparation. 
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