Dilution, by prolonging the time necessary for their being absorbed,
commonly lessens their activity; yet not always; for if a poison which
acts through the blood is also a powerful irritant, moderate dilution
will enable it to enter the vessels more easily: a small dose of
concentrated oxalic acid acts feebly as an irritant or corrosive;
moderately diluted, it quickly enters the blood and causes speedy
death.[52] The effect of mixture may depend also in part on the mere
mechanical impediment interposed between the poison and the animal
membranes. This is particularly obvious when the mass containing the
poison is solid or pulpy; for then the first portions of the poison that
touch the membrane may cause an effort of the organ to discharge the
rest beyond the sphere of action,—if, for example, it is the stomach,—by
vomiting. The effect of mixture in interposing a mechanical impediment
is also well illustrated where the substance mixed with the poison is a
fine, insoluble powder, capable of enveloping its several particles.
Thus it is that small, yet poisonous doses of arsenic may be swallowed
and retained with impunity, if mixed with finely powdered charcoal,
magnesia, and probably cinchona-bark, or the like. Besides diluting and
mechanically obstructing their application, the admixture of other
substances may alter the chemical nature of poisons, and so change their
action.
It is important to keep in view, that the influence of mixture may be
exerted in consequence of the cavity into which a poison is introduced
being at the time filled with contents. Some of the most powerful and
unerring poisons may in such circumstances altogether fail to produce
their usual effect, if speedily vomited. Thus Wibmer notices the case of
a man, who swallowed an ounce and a half of arsenic after a very hearty
meal, had merely a severe attack of vomiting with subsequent colic, and
got quite well in four days.[53] And a still more pointed instance has
been briefly mentioned by Dr. Booth of Birmingham, where an ounce of
corrosive sublimate was swallowed after a full meal without any material
ill consequence, vomiting having been speedily induced.[54]