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. MERCURIAL VAPOURS. It is not the least interesting fact in the history of aërial poisons, that substances, which are found to be extremely slow in their action, or even quite inert, when administered in their solid or liquid state, exert a very rapid and energetic operation when they are presented to the human body in the attenuated form of vapour. This fact is well illustrated by the subtlety and activity of metallic mercury _in the state of vapour_; a substance which, according to the highest authorities, is quite inactive when introduced in its grosser form into the stomach. It is thus that the workmen employed in gilding, silvering looking-glasses, constructing barometers, &c. experience such dreadful effects; that such effects arise from the _metal_ in a state of vapour, and not, as some have supposed, from the _oxide_,[500] is a fact capable of demonstration, for the artists at Birmingham affix an apparatus in their chimneys as a system of economy, in order to collect the mercury, which is always found in its _metallic_ state.[501] From the late interesting experiments of _Mr. Faraday_,[502] it appears that _mercury_ rises in vapour at the ordinary temperature of the atmosphere; the knowledge of which fact will afford a very satisfactory explanation of several phenomena, which were previously unintelligible. _Dr. Hermbstaed_, in the memoir, above mentioned, “on the volatility of substances hitherto considered as fixed bodies,” relates the following curious fact with regard to the volatility of mercury. “At the Royal Manufactory of looking-glasses in Berlin, during a severe winter, the artificers who worked in a room, which had originally served for the process of _silvering_ the glasses, lighted a fire, and thus heated the apartment to between 86° and 96° _Fah._ In a few days the whole of them