The following experiment will at once illustrate the property which some bodies possess of absorbing effluvia from the atmosphere, and prove the influence of heat in again expelling and dissipating them. Pure sand, exposed to a red heat to drive off impurities, was put amidst tainted air. Put into a glass tube and exposed to a spirit lamp, it yielded ammonia or hartshorn,—a product of putrefaction which the sand had undoubtedly absorbed from the tainted atmosphere. Ammonia is a compound of nitrogen and hydrogen, gases which are evolved during the putrefaction of animal materials. The investigation of the means by which persons, merchandize, clothes, letters, &c., may be most speedily and most effectually freed from effluvia, contagion, and other unwholesome impurities, is a most important point, for it relates to the most vital interests of society, commerce, freedom of intercourse, personal liberty, and the safety and health of the community. But from the very important considerations with which the investigation is connected, the merits of the respective means employed for the purpose will not be treated of here, as they deserve a more extended consideration than can be given. In the mean time it would be highly dangerous and impolitic, to adopt any great and rash change in a system so important as quarantine, until the most full and sound inquiry has been made upon the subject. Public safety demands the utmost caution. There may exist great diversity of opinion respecting the nature of the impurities with which merchandize and clothes are sometimes impregnated, on the period during which they retain their activity, and on the means of purification; but it has been often clearly demonstrated, that specific contagious matter, or virus, and effluvia, may be conveyed by these bodies, may be retained for a considerable time, and, on a favourable opportunity, produce very hurtful effects. The impurities may be variously designated, yet their unwholesome tendency is much the same, and it is necessary to adopt provisions to counteract it. CHAPTER XIII. THE PREVENTION OF VITIATED AIR IN CONNECTION WITH THE DISPOSAL OF THE DEAD—OFFALS—CONSTRUCTION OF TOWNS, HOUSES, SEWERS, &C. In the Chapters which have been dedicated to the subject of Vitiated Air, its sources were pointed out in a general manner, and it is intended to consider those usages in society, certain conditions of towns and houses, and some other circumstances, which favour the production of an impure and unwholesome atmosphere, and this will be done with the hope that a knowledge of their hurtful tendency may lead to their correction. The disposal of the dead will be first considered.