MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 13 



through tubes bent several times, if it is drawn in slowly and 

 regularly. 1 



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 is applied too long. 



If the substance cannot be boiled without suffering great 

 change or entirely losing its original nature, fractional steri- 

 lisation 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 proceed 

 in a different way in order to sterilise it in the gelatinous 

 state. It was observed that a temperature of 58 to 62 C. 

 in many cases sufficed 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 gelatinised 

 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.), a number of these germinate, and the new vegetative 

 rods can then be killed by again heating to about 60C. If 

 this process is repeated several times, the gelatinous mass will 

 commonly 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 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 



1 In the so-called Pasteurisation of beer, wine, etc., a merely relative sterilisa- 

 tion is all that is generally aimed at ; that is to say, by a cautious treatment 

 of the liquid at elevated temperatures, it is attempted to check the yeast cells 

 and other micro-organisms to such a degree that they are capable only to 

 a very limited extent of multiplying and producing fermentation. It is only 

 for transportation to a great distance or for preservation of the liquid for a 

 very long time that the attempt is made to kill all living germs. No general 

 rules can be laid down for a treatment of this 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 experiments must always be 

 made with regard to the temperature required and the length of time the 

 operation must last. 



