THE DESTRUCTION OF BACTERIA BY CHEMICALS 111 



Even ordinary soapsuds have a slight bactericidal as well as a marked 

 cleansing effect. The bicarbonate has almost no destructive effect on 

 bacteria. 



Calcium Compounds. Calcium hydroxide Ca(OH) 2 is a powerful 

 disinfectant; the carbonate, on the other hand, is almost of no effect. 

 A 1 per cent, watery solution of the hydroxide kills bacteria which are 

 not in the spore form within a few hours. A 3 per cent, solution kills 

 typhoid bacilli in one hour. A 20 per cent, solution added to equal 

 parts of feces or other filth and mixed with them will completely sterilize 

 them within one hour. 



Effect of Acids. An amount of acid which equals 40 c.c. of normal 

 hydrochloric acid per litre is sufficient to prevent the growth of all 

 varieties of bacteria and to kill many. Twice this amount destroys most 

 bacteria within a short time. The variety of acid makes little difference. 

 Bulk for bulk, the mineral acids are more germicidal than the vegetable 

 acids, but that is because their molecular weight is so much less. A 

 1 : 500 solution of sulphuric acid kills typhoid bacilli within one hour. 

 Hydrochloric acid is about one-third weaker, and acetic acid some- 

 what weaker still. Citric, tartaric, malic, formic, and salicylic acids 

 are similar to acetic acid. Boric acid destroys the less resistant bac- 

 teria in 2 per cent, solution and inhibits the others. 



Gaseous Disinfectants. 



The germicidal action of gases is much more active in the presence 

 of moisture than in a dry condition. 



Numerous experiments have been made with sulphur dioxide gas 

 (SO,'), owing to the fact that it has been so extensively used for the 

 disinfection of hospitals, ships, apartments, clothing, etc. This gas 

 is a much more active germicide in a moist than in a dry condition; 

 due, no doubt, to the formation of the more active disinfecting agent 

 sulphurous acid (H 2 SO 3 ). In a pure state anhydrous sulphur dioxide 

 does not destroy spores, and is not certain to destroy bacteria not in 

 spore form. Sternberg has shown that the spores of the bacillus anthracis 

 and bacillus subtilis are not killed by contact for some time with liquid 

 SO ( liquefied by pressure). Koch found that various species of spore- 

 bearing bacilli exposed for ninety-six hours in a disinfecting chamber 

 to the action of SO 2 , in the proportion of from 4 to 6 per cent, by volume, 

 were not destroyed. In the absence of spores, however, the anthrax 

 bacillus in a moist condition, attached to silk threads, was found by 

 Sternberg to be destroyed in thirty minutes in an atmosphere containing 

 1 volume per cent. As the result of a large number of experiments 

 with SO 2 as a disinfectant it has been determined that an "exposure 

 for eight hours to an atmosphere containing at least 4 volumes per 

 cent, of this gas in the presence of moisture" will destroy most, if not 

 all, of the pathogenic bacteria in the absence of spores. Four pounds 

 of sulphur burned for each 1000 cubic feet will give an excess of 

 gas. 



