158 BACTERIOLOGY. 
The Effect of Acids. An amount of acid which equals 
40 e.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 to 500 
solution of sulphuric acid kills typhoid bacilli within 
one hour. Hydrochloric acid is about one-third weaker, 
and acetic acid somewhat weaker still. Citric, tartaric, 
malic, formic, and salicylic acids are similar to acetic 
acid. Boric acid destroys the less resistant bacteria 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 disin- 
fecting agent—sulphurous acid (H,SO,). Ina 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 
