MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 13 



Nutritive gelatines must be treated with particular care, 

 as they often lose their power of gelatinising if the heat is 

 too great or if it be applied too long. 



If the substance cannot be boiled without suffering great 

 changes or entirely losing its original nature, fractional 

 sterilisation must be resorted to. 



This, for instance, is the case with blood-serum, which is 

 employed in a gelatinous condition in bacteriological studies. 

 This substance, when heated to 100 C., becomes fluid, and 

 does not again solidify, and it is, therefore, necessary to pro- 

 ceed in a different way in order to get it sterilised in the 

 gelatinous state. It was observed that a temperature of 58 

 to 62 C. was sufficient to kill the vegetative bacteria which 

 develop in blood-serum. By this treatment of the substance 

 only the spores of bacteria remain unkilled. If the gela- 

 tinised serum is placed for two or three days in an incubator 

 at a temperature favourable to the development of the spores 

 (30 to 40 C.), the greater number of these germinate, and 

 the new vegetative rods can then be killed by again heating 

 to about 60 C. If this process is repeated several times, 

 the gelatinous mass will remain sterile for an unlimited 

 time. This process, which is also used for the sterilisation of 

 milk, and which was discovered by Tyndall, has been further 

 established by R. Koch. 



A similar method is employed in zymotechnical laboratories 

 for the treatment of nutritive liquids, which, when boiled, are 

 apt to deposit a considerable amount of albuminoid matter, 

 and would thus become comparatively bad nutritive liquids 

 for the alcoholic yeast. 1 



in beer. No general rules can be laid down for a treatment of that kind. 

 The correct procedure depends on the nature of the liquid as well as on 

 the properties of the particular species of yeast, and preliminary experi- 

 ments must accordingly always be made with regard to temperature as 

 well as to the duration of the action of the particular temperature. 



1 Sterilisation is also attempted in practice, namely, for the purpose 

 of introducing wort in a sterile condition by means of closed cooling and 

 aerating apparatus into the fermenting-tuns. It is true that the wort 



