BACTERIA. 57 



may occur, which gradually increase in thickness. The cause 

 of this is commonly a growth of bacteria occurring in such 

 a manner that the single cells lie very close to each other, 

 whilst at the same time the outer gelatinous layers of the 

 cell-wall greatly swell up. During the continued growth 

 of the bacteria the slimy layer increases in thickness, and 

 often assumes certain characteristic forms. Such slimy masses 

 known in sugar manufacture as " frog-spawn " occur 

 both on solid and in liquid media. In a microscopical 

 preparation, this bright, transparent mass of slime is 

 not discernible. If specially treated for instance, by im- 

 mersing the preparation for some minutes in broth at about 

 35 C. the slime-masses or capsules usually absorb colouring 

 matter, and it then becomes possible to observe their form. 



Many bacteria are very resistant to low temperatures. Thus 

 J. FOBSTER, B. FISCHER, MIQUEL, and others found that there 

 are bacteria which multiply very well at 0. Certain species 

 withstand cooling down to 70 C., 110 C., even to 213 C., 

 without being killed (FRISCH, PICTET, and YUNG). On the 

 contrary, a number of bacteria capable of withstanding heat 

 (thermophilous) have been discovered. The bacillus ther- 

 mophilus described by MIQUEL multiplies actively even at 

 70 C., its growth stopping at 42 C. Other species do 

 not develop at lower temperatures than 60 C. In excre- 

 ments of animals several frequently occurring species were 

 discovered, which continued growing even at 75 C., whilst 

 at about 39 C. they ceased multiplying (L. EABINOWITSCH). 

 To these thermophilous species belong several lactic acid 

 bacteria, and probably also the various bacteria occurring in 

 tobacco fermentation. It may also be mentioned that 

 amongst the bacteria of this class a certain micrococcus was 

 shown by F. CORN to be the cause of the spontaneous ignition 

 of moist cotton. 



The bacteria are very sensitive to the action of light, 

 and many of them are quickly killed in direct sun-light. 

 This fact plays an important part in the so-called " spon- 

 taneous purification " of rivers (H. BUCHNER). 



Towards mechanical concussion bacteria and the other 

 micro-organisms behave differently. MELTZER concluded, after 



