42 MICRO-ORGANISMS AND FERMENTATION. 



purposes, an objection based upon numerous experiments 

 has been raised by Miquel, who asserts that many bacteria, 

 when exposed to a temperature of 20 to 22 C., require an 

 incubation of a fortnight before developing distinct colonies 

 in gelatine ; on the other hand, however, there are species 

 which will very soon liquefy the gelatine, thus rendering 

 further observation impossible for the next fortnight. The 

 same is the case with the mould-fungi, which will often 

 spread over the whole plate in a few days. Thus, it becomes 

 necessary to count the colonies at such an early stage when 

 many are not to be seen. An additional drawback to the 

 gelatine plates is, that the development cannot take place at a 

 temperature higher than 23 to 24 C., otherwise the gelatine 

 will become liquid ; but many species of bacteria give a fair 

 development only at considerably higher temperatures. Other 

 species, moreover, do not develop in gelatine at all, but only 

 in liquids. Finally, it is urged as a very material objection 

 to the gelatine-plates, that many of the colonies consist of 

 several species (see p. 31) ; Miquel proved this by intro- 

 ducing the colonies, one by one, into meat-decoction with 

 peptone, and then again preparing plates from these growths. 

 This is in part due to the fact that bacteria, as shown by 

 Petri, often occur in aggregates in the air, and these will 

 either fall directly on to the gelatine-plate or become mixed 

 in the liquid gelatine, where it will always be very difficult to 

 separate the individuals from each other by agitating. 



Hansen's investigations of the air were made between 

 1878 and 1882. The main object of his investigations was 

 to throw light on questions affecting the fermentation indus- 

 tries. As is known, his researches on Saccharomyces apicula- 

 tus (1880) were partly based on work of this nature. Since 

 the question concerned the organisms which occur in brewing 

 operations, the choice of a nutritive liquid was easily made, 

 namely, ordinary wort as employed in breweries. The appara- 

 tus employed consisted either of ordinary boiling flasks closed 

 with several layers of sterilised filter-paper, the contents of 



