This has been treated of in reference to its preparation, sensible and chemical properties, and medical uses, at page 412 of the present volume.* We have to consider it here chiefly in its relations to the present class of medicines. Sulphurous acid is at once a powerful disinfectant and antizymotic; that is, it destroys through its deoxidizing property offensive and noxious effluvia, and prevents their production by arresting completely the fermentative processes in which these effluvia originate. From the earliest records of history, we have evidence of its employment as a disinfectant. it has long been known to have the power of discharging organic colours, and it was much employed in bleaching before chlorine was discovered. it has been proved on trial to correct offensive odours very promptly. Dr. Angus Smith and Dr. Dewar have experimentally demonstrated that it will protect fresh meat from putrefaction, and preserve it in a state perfectly fit for use; and Mr. Crooks, famous for his investigations as to the means .of preventing the cattle-plague, which has recently devastated the pastures and cattle-stalls in some parts of Europe, has conclusively shown that it has the power of arresting fermentation, and of destroying the vitality of the germs on which that process depends. it may not be so efficient to this end as the carbolic acid and other derivatives of coal tar, but, according to Mr. Crooks, it may be advantageously employed as an auxiliary to those agents in checking the progress of pestilence. it is usually employed for these purposes in its gaseous state.

One of the advantages of sulphurous acid gas, as an antizymotic, is its affinity for moisture, by means of which organic germs, which have a strong tendency to attach themselves to the watery vapour of the atmosphere, are brought more completely within the influence of the acid. The objection has been urged against it, that, through its affinity for oxygen, it is converted into sulphuric acid, which materially injures the fabrics which may be exposed to its action; but this disadvantage can readily be guarded against by due precaution. The gas is peculiarly adapted to the disinfection of unoccupied apartments, whether of private dwellings, of hospitals, prisons, ships, etc.; and may be resorted to in some instances when chlorine might be objectionable, in consequence of white-wash on the walls, which, as already stated, forms with the latter a deliquescent chloride, by which the air is kept in a state of dampness.

* A remark made in that place, rather depreciating the value of sulphurous acid as a disinfectant, requires some explanation. it was made at a time 'when experiments had not yet demonstrated its extraordinary powers; and escaped the notice of the author when revising that part of the work, or would have been modified in accordance with his present views on the subject. [Note to the third edition.)

Sulphurous acid, on the contrary, forms with the lime a perfectly dry sulphite, which has to a certain extent the powers of the acid, and therefore continues to exercise a disinfectant influence for a considerable time.

Gaseous sulphurous acid is readily obtained by the combustion of sulphur, a little of which, placed in suitable vessels, and set on fire, will answer for the fumigation of the air to any desirable extent. The excessively irritating property of the gas, and its suffocative effect when inhaled, render it necessary that no living person or domestic animal should be exposed to its action; and fabrics, moreover, which might suffer injury from its decolorizing power, or its chemical action in the state of sulphuric acid into which it passes, should be effectually protected or removed. Happily, the intensity of its effect on the nostrils, even in very small quantity, gives sufficient warning to escape. it should never be employed in the chambers of the sick while occupied.

In the liquid form, which is that of water impregnated by the gas (see page 412), it is comparatively little used; though I have no doubt that it would prove very beneficial, applied as a wash to surfaces to which the germs of contagion or infection might be adhering, or mixed with liquids or solids in which morbific fermentative processes might be going on.