I place these in one category, because they act as disinfectants upon the same principle; and, as all of them have been already fully treated of elsewhere in this work, little more will be required here than to indicate the precise method of using them for disinfecting purposes. They correct foul odours and noxious effluvia of all kinds which are the pure result of organic decomposition, in the same manner as ozone; that is, by oxidizing the elements of these gaseous exhalations, and thereby forming perfectly innocent compounds. But, instead of immediately imparting oxygen, they do so indirectly through their strong affinity for hydrogen, which they abstract from water, setting free the oxygen. This element, being in the nascent state, acts with an energy of combination little short of that of ozone; and thus the oxidizement is effected, which proves so powerfully destructive of colour and smell in organic substances. it is possible that they sometimes act by abstracting hydrogen from the coloured and fetid compounds; but, as the presence of moisture is essential to their full activity, the probability is that their influence is mainly through its preliminary decomposition as above stated. Chlorine is the most efficient of these three bodies, on account probably of its essentially gaseous nature, and the consequent facility of its diffusion wherever foul effluvia can penetrate. The only objections to it are its disagreeable smell, and its irritant property when too largely inhaled; but the former is generally temporary, and when necessarily permanent becomes by habit readily supportable; and the latter may be avoided with the exercise of a little caution. it should be remembered, in the use of chlorine, that it has strong affinities for various substances, especially the metals, even those which resist most other chemical agencies, such as silver and gold; and care should be taken, when it is used, to remove objects of this kind from its sphere of action. Nor should it be employed in connection with lime, used as white-wash, in consequence of forming with it the deliquescent chloride of calcium, by which the apartments would be maintained in a damp state. in relation to its employment as a disinfectant, whether in the gaseous state or in aqueous solution, or in the form of the chlorinated compounds, as chlorinated lime and chlorinated soda, it has been so fully treated of, under the head of the alteratives, as to require little more to be said here. (See pages 372-4-6-8.) it may be well, however, to suggest that the best method, on the whole, of using chlorine for the disinfection of apartments, is to expose, in flasks or in porcelain or glass dishes, a few ounces of a mixture of common salt, black oxide of manganese, and sulphuric acid; the last to be more or less diluted according as a rapid or slow evolution of the gas is required. Wiggers recommends 18 parts (say drachms) of finely ground common salt, and 15 parts of finely powdered black oxide of manganese to be introduced into a flask; then to be added 45 parts of concentrated sulphuric acid and 21 of water, previously mixed, and completely cooled; and lastly the flask to be well shaken. When the escape of chlorine ceases, it may be renewed by the application of a gentle heat, which may be readily effected by placing the flask in heated water. it may be mentioned, in addition, that chlorine cannot be depended on for the destruction of organic germs, unless used in a degree of concentration which might be injurious to the health of man, if not fatal.

Bromine is perhaps next to chlorine in disinfecting powers; for, though liquid at ordinary temperatures, it is extremely volatile, and its vapours are rapidly diffused through the surrounding air, when it is exposed in open vessels. This agent has been fully considered with the alteratives. (See page 392.) Perhaps the most convenient method of using it is to place in the apartment to be disinfected one or more small bottles, with ground glass stoppers, which are to be left open until the fetid odour shall be superseded by that of the bromine, and then closed; to be again opened should their agency be needed.

iodine has also been treated of with the alteratives (page 341). Although much inferior to the two preceding substances, it is not without disinfectant properties. Every one familiar with iodine is well aware that, solid as it is, it is constantly emitting odorous vapours, which will affect the surrounding air to a considerable distance. All that is necessary is to expose it, in the state of coarse powder, or simply in the ordinary form in which it is found in the shops, in one or more plates or saucers, in the situations where its action is wanted. An objection to both bromine and iodine, when compared with chlorine, is their greater costliness.