Tan yards and places in the immediate vicinity, are said to be free from pestilences. The tanners of Bermondsey are said to have escaped the Plague of London, and one person only died in Gutter Lane, where was a tan yard. The tanners of Rome are also stated to have been free from Plague. Dr. McLean refers to the exemption of tanners at Cairo. _Tannin is prejudicial to most vegetables_,--but Dr. Lindley says it is not always so to fungi. "A species of Rhizomorpha is often developed in tan pits." I should imagine that neither plants nor insects would be found very abundantly, where tannin prevails; yet we find that the gall-nut is formed for the protection of an insect from injury by weather, and as a temporary means of sustenance. The custom of fumigating with odoriferous substances, does not therefore appear upon this view of the matter to be destitute of importance; indeed, the universal practice stamps it at once, as an efficacious remedy for the purposes of disinfection. The introduction of chlorine fumigation, seems to have superseded, in a great measure, the use of fragrant herbs and woods; and it is questionable whether the substitution be altogether desirable or {184} advantageous. Many scents may be agreeably and usefully employed, with much less chance of annoyance to the patient, and considerably less injury to articles of furniture, &c. The fumigations of sulphurous acid and chlorine are, perhaps, more adapted as disinfectants in uninhabited apartments;--their power to destroy vegetation, is well known. They have been used, chiefly, with the idea of neutralizing gaseous exhalations, particularly chlorine, as it tends to combine with hydrogen, to form hydrochloric acid, and then to unite with ammoniacal matters, forming hydrochlorate of ammonia. This, supposing noxious or pestilential effluvia consisted of the ammoniacal exudations variously combined, was an exceedingly efficacious method of rendering them inert; but as we feel convinced that no ammoniacal compound could possibly be the cause of infection, we must look to the influence these gases possess over other forms of matter, and as they are so destructive, even in minute quantities, to vegetable existence, it is possible that their beneficial effects may be due to this property. The immediate neighbourhood of gas works is prejudicial to vegetation, I imagine, from the amount of sulphurous vapours, and to this has been attributed the exemption of persons employed in these works. Many other instances might be cited of a similar nature. I have now to speak of medicinal agents, and here comes a considerable difficulty. {185}