PREVENTIVE MEDICINE 205 



dry, the outside of the ricks showed a brownish appearance as if 

 the hay had been boiled.* 



CHEMICAL DISINFECTANTS. 



Chemicals act as disinfectants by reason of (1) their oxidising 

 power; (2) their reducing power; and (3) their corrosive and 

 coagulant effect on the albumen of bacteria and parasites. 



The conditions under which the various chemicals are used 

 affect to a very marked degree their potency. All solutions of 

 disinfectants should be made with clean water. Moderately soft 

 water, that is water not containing more than 12 parts of calcium 

 carbonate per 100,000, is much to be preferred to a hard water. 

 This is especially important when the disinfectant has a cleans- 

 ing action as is the case with Liquor Cresolis Saponatus and 

 similar preparations. In making a solution of a desired strength 

 it is important that the quantities of water and the disinfectant 

 are accurately measured and not guessed. No other chemicals 

 must be added than the one it is desired to use. The combination 

 of two or more disinfectants, so far from increasing the potency of 

 the solution, may bring about chemical reaction with the production 

 of inert new substances. The vessel in which the solution is made 

 must be clean, and the reagent used must be thoroughly mixed with 

 the water and not merely added to it. In selecting a disinfectant 

 careful thought should be given to the purpose for which it is to 

 be used. Some disinfectants are rendered inert, or nearly so, in 

 the presence of much albuminous material, such, for instance, as 

 perchloride of mercury and potassium permanganate. Others are 

 especially suitable for the disinfection of floors, walls, &c., which if 

 used on harness, clothing, and the like would speedily destroy them. 

 Formaldehyde, for example, has a tanning or drying effect on 

 leather. Bleaching powder, one of the most powerful and useful 

 disinfectants the veterinary surgeon can use, is destructive to 

 harness and clothing if used too strong. The time factor is 

 important, and a sufficient period must be allowed to elapse for the 

 disinfectant to do its work. Temperature has a great influence 

 upon the potency of disinfectants. A solution of a disinfectant 

 (except bleaching powder) in hot water is much more powerful than 

 one made with cold water " Some idea of the magnitude of the 

 effect of temperature may be gathered from the fact that with 

 metallic salts the mean velocity of disinfection increased two to 

 fourfold for a rise in temperature of 10 C, while with phenol it 

 * (Report of Foot-and-Mouth Disease in Ireland, 1912. Cd., 7103.) 



