§ VI.] Culture of Bacteria. Sterilisation. 63 



periment is made with sound ripe seeds of the ordinary garden 

 cress, Lepidium sativum, the same result is obtained ; they ger- 

 minate if they are taken from the alcohol after four weeks' time, 

 and washed and sown. The spores of the Bacillus and the 

 seeds of the cress agree in being enveloped all round in a gela- 

 tinous membrane into which the alcohol cannot penetrate, and 

 thus the protoplasm, which in the cress-germ would otherwise 

 be certainly killed at once, remains unattacked. 



But the application of poisons to cultures for purposes of 

 sterilisation is attended with great inconveniences in all the many 

 cases in which they must be got rid of again that they may do no 

 injury to the culture itself. New impurities may be introduced in 

 the process of washing the vessels and the rest of the apparatus. 



Hence much the most practical mode of sterilisation consists 

 in the application of extremely high temperatures, which must 

 exceed 100° C, if the object is to kill any spores that may pos- 

 sibly be present ; in dry vessels it is best to raise it to 1 20- 

 150° C. In the sterilising of fluids a heat of even 100° C. may not 

 always be possible for practical reasons, as, for example, when it 

 is necessary to avoid the coagulation of the albuminous sub- 

 stances dissolved in the fluid. Since most vegetating cells are 

 killed by a temperature of 50-60° C, the plan suggested by 

 Tyndall (33) is the most effective ; the fluid is allowed to stand 

 till whatever germs it contains begin to grow ; if it is then 

 heated to 60-70° C. and the process repeated at an interval of 

 two days, the fluid will be in most cases free from Bacteria, 

 always presupposing of course that the plug which closes the 

 vessel is compact and clean. 



Lastly, in practical life all that is usually required is to 

 render harmless any germs that may be present by preventing 

 their further development, whether they continue capable of it 

 or not. Here, too, complete destruction would be best and 

 most desirable ; but the use of most poisons in the state of con- 

 centration which is most certainly fatal, or that of a certainly 

 fatal degree of heat, would also ordinarily lead to the destruc- 



