64 MICRO-ORGANISMS AND FERMENTATION. 



When samples of the air contents are to be sent from one 

 place to another, these air filters will answer the purpose. On 

 receipt of a sample, the sand may be washed into gelatine or, 

 preferably, into sterilised water. After vigorously agitating 

 the water, it is added in drops to flasks containing nutritive 

 liquid, or it may be used in plate-cultures. 



When samples of air are to be sent to the author's laboratory 

 short cylindrical glasses are used, having india-rubber stoppers, 

 which project well beyond the mouth of the glass. The latter 

 are half-filled with sterilised water or with the nutritive liquid 

 in actual use. When the glass is opened at its destination the 

 stopper is placed by it with the wet end turned upwards, 

 care being taken, of course, that this is not touched. A 

 suitable time having elapsed, the stopper is replaced and tied 

 down. 



Miquel has raised an objection to the employment of 

 gelatine plates for this purpose, based upon numerous experi- 

 ments. He asserts that many bacteria, when exposed to a 

 temperature of 20 to 22 C., require a fortnight's incubation 

 before developing distinct colonies in gelatine ; on the other 

 hand, there are species which very soon liquefy the gelatine, 

 thus rendering further observation impossible. The same is 

 the case with the moulds, which often spread all over the 

 plate in a few days. Thus, it becomes necessary to count 

 the colonies at so early a stage that many of them are not yet 

 visible. 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 liquefy, but many 

 species of bacteria present a characteristic 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. Miquel 

 proved this by introducing 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 the 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 would 



