42 MICRO-ORGANISMS AND FERMENTATION. 



FRANKLAND, MIQUEL, and PETRI, use porous solid substances 

 for the filtration of air for analytical purposes ; as, for example, 

 powdered glass, glass-wool, sand, sugar, etc. The sand-filter 

 employed by PETRI is 3 cm. long and 1/8 cm. wide. It is 

 filled with sand, previously ignited, the size of the grains being 

 from 0'25 to 0'5 mm. Two such sand-filters are placed one 

 behind the other in a glass-tube. The first filter should retain 

 all the dust-particles containing germs, w^hilst the other filter 

 should remain sterile, and thus serves as a control. The sand 

 charged with germs is distributed in shallow glass dishes and 

 covered with liquid gelatine. The germs contained in the 

 dust-particles will then develop colonies in the gelatine. 



When samples of air 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, prefer- 

 ably, 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. 



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 will very soon liquefy the 

 gelatine, thus rendering further observation impossible. The 

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

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

 count the colonies at an early stage when many 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 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. 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 



