146 BACTERIA IN AIR, SOIL, AND WATER. 



Formalin gas has only a limited application ; it has little 

 effect on dry organisms, and in the case of wet organisms must, 

 in order to be effective, probably become dissolved so as to give 

 the moisture a proportion analogous to the strengths given above 

 with regard to the vapour. 



Sulphurous Acid. — This substance has long been in use, 

 largely from the cheapness with which it can be produced by 

 burning sulphur in the air. An atmosphere containing 98 per 

 cent will kill the pyogenic cocci in two minutes if they are wet, 

 and in twenty minutes if they are dry ; and anthrax bacilli are 

 killed by thirty minutes' exposure, but to kill anthrax spores an 

 exposure of one to two hours to an atmosphere containing 1 1 

 per cent is necessary. For a small room the burning of about 

 a pound and a half (most easily accomplished by moistening the 

 sulphur with methylated spirit) is usually considered sufficient. ' 

 It has been found that if bacteria are protected, as by being 

 in the middle of small bundles of clothes, no effect is produced 

 even by an atmosphere containing a large proportion of the 

 sulphurous acid gas. The practical applications of this agent 

 are therefore limited. 



Potassium Permanganate. — The action of this agent very 

 much depends on whether it can obtain free access to the bac- 

 teria to be killed or whether these are present in a solution con- 

 taining much organic matter. In the latter case the oxidation 

 of the organic material throws so much of the salt out of action 

 that there may be little left to attack the organisms. Koch 

 found that to kill anthrax spores a 5 per cent solution required 

 to act for about a day; for most organisms a similar solution 

 acting for shorter periods has been found sufficient, and in the 

 case of the pyogenic cocci a i per cent solution will kill in ten 

 minutes. There is little doubt that such weaker solutions are of 

 value in disinfecting the throat on account of their non -irritating 

 properties, and good results in this connection have been obtained 

 in cases of diphtheria. A solution of i in 10,000 has been found 

 to kill plague bacilli in five minutes. 



Carbolic Acid. ■ — Of all the aromatic series this is the most 

 extensively employed antiseptic. All ordinary bacteria in the 

 vegetative condition, and of these the staphylococcus pyogenes 

 is the most resistant, are killed in less than five minutes by a 

 a to 3 per cent solution in water, so that the 5 per cent solution 



