62 Lectures on Bacteria. [^ vi. 



fruits, neutralised, if necessary, and dissolved in not too con- 

 centrated (about 10 per cent.) watery solutions or in gelatine, 

 are, as a rule and in accordance with general experience, good 

 nutrient substrata ; the special choice must be made by experi- 

 ment in each case. Fresh urine has been repeatedly used with 

 success by French observers. The serum of blood has been 

 found to be a very suitable substance, and is almost the only 

 one that can be used in the cultivation of some parasitic forms, 

 especially if made stiff by being heated up to 60-70° C. after the 

 mode of proceeding described by Koch. 



Among the very first requisites are the securing the purity of 

 the species under cultivation, the absence of unintentional ad- 

 mixtures, on which point some practical hints were given in a 

 former lecture (pp. 34 and 41), and the perfect control of the 

 continued purity of the cultures. The possibility of different 

 species displacing each other has been already discussed (p. 32). 



To obtain purity of a culture as well as for other practical 

 purposes, it is often necessary to effect the entire destruction or 

 death of germs present in it. In the conduct of cultures there 

 is the special risk of these germs adhering to the apparatus to 

 be employed, vessels, nutrient substances, &c., and they must 

 be killed in order to provide for the purity of the culture. This 

 process of destruction is known as sterilisation, an expression 

 introduced by the school of Pasteur. 



Bodies poisonous to protoplasm, such as acids, corrosive sub- 

 limate, &c., if sufficiently concentrated will usually effect the de- 

 sired result, where the object is only to destroy, of course on the 

 one condition that they are able to force their way into the proto- 

 plasm which is to be killed. This is the case in most poisons 

 but not in all. Absolute alcohol is a poison which is imme- 

 diately fatal to protoplasm, and it must therefore kill the proto- 

 plasm of endosporous Bacilli, if it reaches them. Nevertheless, 

 the spores of Bacillus Anthracis, as Pasteur discovered, and no 

 doubt also those of other endosporous species retain their vitality 

 after lying several weeks in absolute alcohol. If the same ex- 



