MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 15 



operator, it is important to ascertain their proper degree of 

 dilution. 



Investigations having for their object the determination of 

 the behaviour of the various species of micro-organisms to poisons 

 have proved that it varies greatly in different cultures of one 

 and the same species, not only for the spores, but also for 

 vegetative forms. A young culture behaves differently from 

 an old one, and the same applies to individuals belonging 

 to one and the same culture. In all experiments of this 

 kind, organisms which have been tested with some disinfec- 

 tant should afterwards be allowed the most favourable con- 

 ditions of growth, otherwise they will not develop, even 

 though they are alive and capable of development. In such 

 cases, the ordinary temperature of the room and solid nutrient 

 substrata are not sufficiently favourable : it is also necessary to 

 allow ample time for the observation of such growths before 

 definitely deciding whether they are dead or not ; in fact, it 

 often happens that they have merely been somewhat checked 

 in their development, and that they may develop again, after 

 some time, with their full vigour. Furthermore, the tempera- 

 ture and the medium in which the organisms are present 

 when the disinfectant is employed may be of some importance. 

 Before testing a culture thus treated, great care must be taken 

 to previously free it from all remains of the disinfectant by 

 washing and dilution. 



As early as 1839, SCHWANN stated that yeast cells die 

 under the influence of certain chemicals, the fermentation 

 also coming to a stop. This discovery laid the foundation 

 of the doctrine of antiseptics. 



The first information on this subject we owe to ROBERT KOCH. 

 These researches were continued by GRUBER and others. 



KOCH examined several poisons not only with reference to 

 the degree of concentration requisite for destroying bacteria 

 and spores of bacteria, but also with a view of ascertaining the 

 particular quantity necessary for checking the micro-organ- 

 isms in their power of development in suitable nutritive 

 solutions. 



The results obtained by KOCH may briefly be summarized as 

 follows : Carbolic acid was found to be a less efficient disinfec- 



