It should at the outset be understood that we desire in practical disinfection to inhibit or kill micro-organisms without injury to, or destruction of, the substance harbouring the germs for the time being. If this latter is of no moment, as in rags or carcasses, burning is the simplest and most thorough treatment. But with mattresses and beddings, bedclothes and garments, as well as with the human body, it is obvious that something short of burning is required.
1. From the earliest days of bacteriology heat has held a prominent place as a disinfector. But it is only in comparatively recent times that it has been fully established that _moist heat_ is the only really efficient form of heat disinfection. Boiling at atmospheric pressure (100° C.) is the oldest form of moist heat disinfection, and because of the simplicity of its application it has gained a large degree of popularity. But it must not be forgotten that mere boiling (100° C.) may not effectually remove the spores of all bacilli. Besides, boiling is not applicable to furniture, mattresses, and such-like frequently infected objects. For many of these _hot-air ovens_ were used in the early days. But it was found that such disinfection was no disinfection at all, for not only did it leave many organisms and spores untouched, but the degree of temperature was rarely, if ever, uniform throughout the substance being treated.
The failures following in the track of these methods were an indication of the need of some form of moist heat, viz., steam.
Here it will be necessary to digress for a moment into some of the characters of steam. When water is heated certain molecular changes take place, and at a certain temperature (100° C., 212° F.) the water becomes steam, or _vapour_, and on very little cooling will condense. But if the vapour is heated, it will become practically a gas, and will not condense until it has lost the whole of the heat, _i. e._, the heat of making water into vapour plus the heat of making vapour into gas. A gas proper is, then, the vapour of a liquid of which the boiling-point is substantially below its actual temperature. But we know that the temperature at which it boils depends upon the pressure to which it is subjected (Regnault's law). Hence in reality "steam at any temperature whatever may be a vapour proper, provided the pressure is such as prevents the liquid from boiling below that temperature." In such a condition of vapour it is termed _saturated steam_. But if it is at that same pressure further heated, it becomes practically a gas, and is called _superheated steam_. The former can condense without cooling; the latter cannot so condense at the same pressure. Saturated steam condenses immediately it meets the object to be disinfected, and gives out its latent heat; superheated steam acts by conduction, and not uniformly throughout the object. Its advantage is that it dries moistened objects. As a disinfecting power, superheated steam is much less than saturated steam. There is one further term which must be defined, namely, _current steam_. This is steam escaping from a disinfector as fast as it is admitted, and may be at atmospheric or higher temperatures. The disinfecting temperature which is now used as a standard is _an exposure to saturated steam of 115° C. for fifteen minutes_.