168 ANTISEPTICS AND DISINFECTANTS. 
germicidal agent ; but we may carry the dilution still further, to be 
on the side of safety, by inoculating a second tube containing the 
same amount of sterile bouillon from the first, carrying over in the 
same way one or two ése. We will still be very sure to have a 
considerable number of thé microdrganisms to test the question of 
the destruction of vitality. Instead of bouillon we may use liquefied 
flesh-peptone-gelatin, which gives us the same advantage as to dilu- 
tion of the disinfecting agent; and after inoculating two tubes -as 
above indicated, we may make Hsmarch roll tubes by turning them 
upon a block of ice. The development of colonies will show that 
there was a failure to disinfect; their absence, after a proper inter- 
val, will be evidence of the germicidal action of the agent employed. 
Koch’s Method.—In 1881 Koch published his extended experi- 
ments made to determine the germicidal power of various chemical 
agents as tested upon anthrax spores. His method consisted in ex- 
posing silk threads, to which the dried spores were attached, in a 
solution of the disinfecting agent, and at intervals transferring one 
of these threads to a solid culture medium. The precaution was 
taken to wash-the thread in distilled water when the agent tested was 
supposed to be likely to restrain development. In these experiments 
a standard solution of the disinfecting agent was used, and the time 
of exposure was varied from a few hours to many days. 
The Writer’s Method.—In the writer’s experiments, made in 
1880 and subsequently, a different method has been adopted. The 
time has been constant—usually two hours—and the object has been 
to find the minimum amount of various chemical agents which 
would destroy the test organisms in this time; and instead of sub- 
jecting a few of the test organisms attached to a silk thread to the 
action of the disinfecting agent, a certain quantity of a recent cul- 
ture—usually five cubic centimetres—has been mixed with an equal 
quantity of a standard solution of the germicidal agent. Thus five 
cubic centimetres of a 1: 200 solution of carbolic acid would be 
added to five cubic centimetres of a recent culture of the typhoid 
bacillus, for example, and after two hours’ contact one or two ése 
would be introduced into a suitable nutrient medium to test the 
question of disinfection. In the case given the result obtained 
would be set down as the action of a solution of carbolic acid in the 
proportion of 1: 400, for the 1 : 200 solution was diluted by the addi- 
tion of an equal quantity of the culture. 
Other experimenters have adopted still a different method. In- 
stead of using a considerable and definite quantity of a culture con- 
taining the test organism, they introduce one or two ése from such 
aculture into a solution containing a given proportion of the disin- 
fectant ; then after exposure for a given time the nutrient medium is 
inoculated. 
