ANTISEPTICS AND DISINFECTANTS. 159 



microorganism, 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 microor- 

 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 seen at 

 the end of twenty-four hours. Another most important point is the 

 fact that some of the disinfecting 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 microorganisms, 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 ose into five to ten cubic centimetres of bouillon, 

 the dilution will usually be beyond the restraining influence of the 



