PREVENTIVE MEDICINE 217 



useless. A chemical reaction takes place with the generation of 

 heat and the liberation of formaldehyde gas. The amount of gas 

 formed, and consequently the efficiency of the method, depends very 

 greatly on the relative proportions of the solution of formaldehyde 

 and the permanganate. The best results are obtained with six 

 parts by weight of the 40 per cent, solution to five parts of the 

 crystals (Dorset). A considerable amount of the formaldehyde is 

 in any case destroyed, but if this proportion is used about 50 per 

 cent, of the formaldehyde is converted into gas available for 

 effective action. For disinfecting 1000 cubic feet of space 20 

 ounces of formaldehyde and 16$ ounces of permanganate are 

 required. 



The permanganate should be placed in a bucket on the stable 

 floor, or, if there is a wood floor placed on bricks, the solution 

 poured on and the room closed and sealed. Twelve hours should 

 be allowed to elapse before the gas is allowed to escape. 



To get rid of the irritating vapour after disinfection is completed 

 blow ammonia through the keyhole, this forms on combination with 

 the formaldehyde hexamethylenetetramine. 



Standardisation of Disinfectants. With the large number of 

 disinfectants now on the market it is very desirable to know their 

 relative germicidal action. This is especially necessary with the 

 tar preparations, the composition of which are so variable. The 

 germicidal action of pure carbolic acid or phenol has been carefully 

 determined under conditions of temperature, period of exposure, 

 nature of the medium containing the bacteria, &c. 



It is therefore taken as the unit of comparison, and other dis- 

 infectants are tested under identical standardised conditions so that 

 their relative strengths compared with carbolic acid may be deter- 

 mined. Thus if a given strength of carbolic acid, say a 1 per cent, 

 in distilled water, be taken as 1, then other disinfectants of a 1 per 

 cent, strength will be either greater or less than 1 according to 

 their potency. 



The bactericidal potency of disinfectants is affected greatly by 

 the presence or absence of organic matter, so that the tests are made 

 without organic matter and in a standard nutrient broth. The time 

 of exposure, the bacteria to be destroyed, the temperature, the 

 medium holding the bacteria, and the other conditions remain con- 

 stant, and are standard conditions. The disinfectant to be tested is 

 then used in various dilutions until it is found what dilution will 

 destroy the bacteria in the same time as a given strength of phenol, 

 and under the same standard conditions. Suppose it is found a 

 certain disinfectant in a solution of 1-1000 will do the same work 



