Substances which can prevent infectious diseases from spreading by destroying their specific germs are called disinfectants. These disinfectants can kill pathogenic germs. Heat is a most powerful agent in killing-germs, therefore anything which is subjected to prolonged boiling becomes sterile or germ-free. For purification of clothes and bedding, heat is the best agent, either by boiling them in water or by placing them in a hot-air chamber. The usual arrangement is a furnace with the smoke shaft passing under or on one side of a brick chamber and with a hot-air blast from a shaft running through or under the fire into the chamber itself, or into a passage below it, whence it passes into the chamber through a valve; an exit for the hot-air is provided at the top of the chamber, the clothes are suspended in the chamber, at a little distance from the walls. Various kinds of ingenious apparatus have been recently contrived and are used. Steam disinfecting chambers are necessary for the disinfection of clothes, &c., of a large population, and all large towns and railway stations should have them. High pressure steam in an apparatus contrived for the intermission of its pressure is found to give the best heat penetration to large non-conducting articles such as bedding. Fumigation by burning sulphur or chlorine is a very useful method for disinfection of rooms. Large bonfires of sulphur may also have a beneficial effect on the air.