STEEILISATION 85 



the number that do not fall into the dish, and which are scattered over 

 the walls and various objects in the room. It cannot be expected that 

 these dishes of disinfectant have the power of luring the bacteria from 

 every part of the room into the carbolic acid, as serpents are said to 

 lure birds to their destruction, by the power of fascination. 



If we remember that actual contact is necessary, the absurdity of 

 this idea is evident. The same applies to solid disinfectants meant to 

 be hung on the walls of a room. From these, it is supposed, there 

 emanates an odour deadly in its effect to all the germs in the neighbour- 

 hood. It may at once be stated that no gas is a disinfectant, and still 

 less is an odour, however much it smacks of coal tar. Obviously, 

 therefore, if we wish to purify a room, our only chance consists in 

 spraying the walls, the floor, and the objects in the room, with a reliable 

 liquid disinfectant. This tends to purify the places from whence the 

 air derives its supply of bacteria. In course of time, the bacteria that 

 were in the air when the spraying took place, will fall under the 

 influence of gravity, and be subjected to the influence of the disin- 

 fectant. This will tend to purify the room, but, as seen in what 

 follows, bacteria have a high degree of resistance, especially in the 

 spore condition, and the spraying must be thorough, and, if possible, 

 renewed several times after definite intervals. As to the choice of a 

 liquid, regard must be paid to the following conditions : 



1. Efiiciency as a disinfectant. 



2. If efiicient, the lowest percentage of solution in which it is 



effective. 



3. Cost. 



4. Effect on the substances to be sterilised. 



We shall consider these again, after we have dealt with the various 

 disinfectants. 



Another method of sterilising the air in a large chamber is to filter 

 it as it enters. This method depends on the fact that there are various 

 substances which allow air to pass through, but which retain the 

 enclosed germs. Cotton wool is a filter of this description. A cotton 

 wool filter is employed to purify the air admitted to the sterilised wort 

 in an apparatus for the pure cultivation of yeast. Such filters, however, 

 can be considered reliable only provided the filter be dry, for if wet, 

 the entangled bacteria that the cotton wool has stopped will grow 

 through the filter, and thus contaminate the atmosphere which it is 

 desired to purify. 



Another method of sterilising the atmosphere of a room, is to allow 

 the air, as it comes in, to impinge against a wet surface before entering 



