762 A MANUAL OF BACTERIOLOGY 



penetration make it less efficient in practice. To obtain 

 even the poor efficiency which is its maximum possible it 

 is necessary for the air to be damp and the room most 

 carefully sealed, and in these conditions it is often more 

 injurious to the objects under treatment than to the 

 bacteria against which it is directed. One of the most 

 efficient methods of applying sulphurous acid disinfection 

 is by means of the Clayton apparatus. The gas is gene- 

 rated by burning sulphur in a current of air at a high 

 temperature, and contains, in addition to S0 2 , traces of 

 higher oxides of sulphur. It is also a very efficient 

 vermin-killer, destroying rats, cockroaches, bugs, fleas, 

 flies, etc. 



Alkalies and soaps. The degree of alkalinity of a solu- 

 tion affects, but does not by itself altogether determine, 

 its germicidal power, which is also dependent on the nature 

 of its metal. The hydrates of thallium, lithium, barium, 

 calcium, potassium, sodium, and ammonium have widely 

 different efficiencies, roughly in the order named. For 

 practical purposes only those of potassium, sodium, and 

 calcium need be considered. 1 They exhibit notably the 

 characteristic of all disinfectants in that they work much 

 more vigorously in hot than in cold solution. It is to the 

 hydrates or alkaline carbonates of potassium and sodium 

 that the soaps owe such power as they possess against 

 naked organisms. The relative efficiency of soaps in 

 practical disinfection may be understated by the results 

 of comparative experiment on laboratory cultures because 

 the resistance of the microbe itself to disinfection by 

 chemical substances, and, indeed, by other agencies, may 

 be small compared with the resistance offered by the 

 envelope of grease or greasy dirt, derived from perspira- 

 tion, pus, fat, and the oily grime which pervades cities 



1 See Forrest and Hewlett, Journ. Hoy. Army Med. Corps, February, 

 1904. 



