198 Theory and Practice of Dipping 
is to be referred, in a very great degree, to the increased wetting power 
resulting from the addition of an emulsion. It was proved that by the 
addition to the sodium arsenite solution of a certain quantity of alcohol, 
which would also facilitate wetting, an increased killing power was 
obtained. Nevertheless, the increased killing power of an emulsion 
dip probably cannot be completely attributed to increased wetting 
power alone, it is probable that certain physical properties of the emul¬ 
sion come into play. 
The Action of the Emulsion. 
Under the sirpposition that the tick is destroyed by the arsenic 
which it absorbs through its own cuticle, the enhanced action of a dip 
containing an emulsion is readily explained by the greater wetting 
power of such a dip. With the more perfect wetting, the absorption 
of arsenic would be considerably facilitated. But it has been shown 
that the tick obtains most, if not all, of the lethal dose of poison from the 
skin of the host; and we explain the enhanced action of the emulsion- 
dip in the following manner: 
A tick attached to the skin of an ox is a more or less spherical body, 
difficult to wet, applied to a flat surface—the skin of the ox—likewise, 
on account of its greasy nature, not easily wetted. It is obvious that 
under such conditions, a plain aqueous solution would tend to form an 
‘air gap’ or bridge surrounding the point of contact between tick and 
host (Fig. 2, A). 
If however the surface tension of the dipping-fluid is sufficiently 
small no such ‘gap’ would form, for the capillary attraction would 
