ANTISEPTICS AND DISINFECTANTS. 167 
microérganism, and then testing the question of loss of vitality by 
culture experiments or by inoculations of infectious disease germs 
into susceptible animals. 
The test by cultivation is the most reliable, but in making it 
several points must be kept in view. Naturally the conditions must 
be such as are favorable for the growth of the particular microér- 
ganism which serves as the test ; and we must allow a considerable 
time for the development of the test organism, for it often happens 
that its vital activity has been weakened without being completely 
destroyed, and that growth will occur after an interval of several 
days, while in the control experiment it has perhaps been séen at 
the end of twenty-four hours. Another most important point is the 
fact that some of the disinfectinz agent is necessarily carried over 
with the test organisms when these are transferred to a nutrient 
medium to ascertain whether they will grow, and this may be in 
sufficient amount to restrain their development and lead to the mis- 
taken inference that they have been killed. This is especially true 
of mercuric chloride, which restrains the development of spores in 
very minute amounts. Spores which have been subjected to its ac- 
tion in comparatively strong solutions, when transferred to a culture 
medium may fail to grow because of the restraining influence of 
the mercuric chloride carried over at the same time. For this rea- 
son liquid cultures are to be preferred in experiments of this kind. 
When the test organisms are planted in a solid culture medium the 
chemical agent is left associated with them; in a liquid culture, on 
the other hand, it is diluted, and the microérganisms, being distri- 
buted through the nutrient medium, have the disinfecting agent 
washed from their surface. In the case of mercuric chloride, how- 
ever, the experiments of Geppert show that the agent is so attached 
to spores which have been subjected to its action that ordinary 
washing does not suffice. Moreover, spores which have been ex- 
posed to the action of mercuric chloride without being killed are re- 
strained in their growth by a much smaller proportion of the corro- 
sive sublimate than is required for spores not so exposed—according 
to Geppert, by 1 part in 2,000,000. Geppert therefore proposes, in 
experiments with this agent, to neutralize the mercuric chloride 
which remains attached to the test organisms by washing these in 
a solution of ammonium sulphide, by which the sublimate is preci- 
pitated as an inert sulphide. 
With most agents simple dilution will serve the purpose of pre- 
venting an erroneous inference from the restraining influence of the 
chemical agent being tested. If we carry, by means of a platinum 
loop, one or two 6se into five to ten cubic centimetres of bouillon, 
the dilution will usually be beyond the restraining influence of the 
