Under this division we include all those deleterious substances which
can be administered through the medium of the atmosphere.
Those gases, the respiration of which occasions death by the negative
operation of excluding oxygen, are not ranked under the class of
poisons, for the history of such bodies involves physiological views
peculiar to themselves, and belongs more correctly to the subject of
suffocation, under which head it has already met with full
consideration, _vol._ 2, _p._ 48.
_Aërial poisons_ are of very undefined extent, and their history is
involved in considerable obscurity. Every poison, capable of
volatilization, may be admitted into the division; and even those
substances which are generally regarded as fixed, may be mechanically
suspended in the air, and thus produce their effects on the living
system, through the medium of the lungs, stomach, or nerves. In the
present state of our knowledge, we have, perhaps, only an imperfect idea
of the distinction between a fixed and a volatile body. A very
interesting paper on this subject was read before the Royal Academy of
Berlin, by _Professor Hermbstaed_,[497] in which he observes that,
generally speaking, we might consider all bodies as volatile, as it is
most probable that, could we produce a sufficient degree of heat, no
substance could resist it. The professor also states that many bodies,
hitherto considered as fixed, are actually volatilized at the
temperature of boiling water; such he found to be _lime_, _baryta_,
_strontia_, and _potass_. We apprehend, however, that the professor has,
in these instances, mistaken a phenomenon for _volatility_, which it is
highly important to distinguish from it, viz. _the elevation of a
certain portion of a fixed body, by the carrying power of a vapour_;
thus, fixed oil may, in a minute proportion, be carried up with the
steam of water. Certain bodies, however, which have been long considered
as perfectly fixed at the ordinary temperature of the atmosphere, have
been lately discovered to undergo a slow and almost imperceptible
evaporation under such circumstances; and the discovery has led to a
very satisfactory solution of several problems which were previously
unintelligible. We shall adduce a striking exemplification of this
truth, under the consideration of mercurial vapours.
The substances, included under the head of _Aërial poisons_, may be
conveniently arranged in two orders, viz.
I. Those, whose particles exist mechanically suspended in the
atmosphere.
II. Those, which are presented to us in a _vaporous_ or _gaseous_
form.
Of the first division the various arts will furnish ample illustration,
as for instance the occupations of the colour-maker, plasterer,
cotton-spinner, dry-grinder,[498] stone-cutter, hatter, furrier, miller,
&c. &c. In all of which a subtle matter is given off, which becoming
mechanically suspended in the air, penetrates the structure of the
pulmonary organs, and excites disease, and even death.[499] In
illustration of the second division, we have the trades of
water-gilders, acid manufacturers, night-men, bleachers, and various
others, many of which have been already noticed under the medical and
chemical consideration of nuisances, _vol._ I, _p._ 330.
In the present chapter we cannot attempt an enumeration of every
substance which may act as an aërial poison; we shall confine our
attention to the history of a few bodies which are calculated to afford
general elucidation, and are likely to become objects of forensic
interest.