EXAMINATION OF AIR, SOIL, AND WATER. 141 



The simplest method for examining the organisms in air consists 

 in exposing plates of glass or microscopic slides coated with 

 glycerine, or with a mixture of glycerine and grape sugar, which is 

 stable, colourless, and transparent. Nutrient gelatine spread out on 

 glass plates may be exposed to the air for a certain time, and then 

 put aside in damp chambers for the colonies to develop. Sterilised 

 potatoes, prepared in the usual way, may be similarly exposed. In 

 both the last- mentioned methods separate colonies develop, which 

 may be isolated, and pure cultivations carried on in various other 

 nutrient media. Nutrient gelatine has also been employed in the 

 special methods of Koch and Hesse. 



Koch's Appouratus. — This consists of a glass jar, about six inches 

 high, the neck of which is plugged with cotton-wool. In the 

 interior is a shallow glass capsule, which can be removed by means 

 of a brass Kfter. The whole is sterilised by exposure to 150° C. 

 for an hour in the hot-air steriliser. The nutrient gelatine in a 

 stock- tube is liquefied, and the contents emptied into the glass 

 capsule. The jarSs exposed to the air to be examined for a definite 

 time, the cotton -wool plug replaced, and the apparatus set aside for 

 the colonies to develop. 



Hesse's Apparatus. — The advantage of this apparatus is that it 

 enables the experimenter to examine a known volume of air. A 

 glass cylinder, 70 cm. long and 3"5 cm. in diameter, is closed at one 

 end by an india-rubber cap, perforated in the centre. Over this fits 

 another cap, which is not perforated. The opposite end of the 

 cylinder is closed with a caoutchouc stopper, perforated to admit 

 a glass tube plugged with cotton-wool. The tube can be connected 

 by means of india-rubber tubing with an aspirating apparatus, 

 which consists of a couple of litre-flasks, suspended by hooks from 

 the tripod-stand which supports the whole apparatus. The cylinder, 

 caps, and plug are washed with solution of carbolic acid, and 

 then with alcohol. After being thus cleansed, 50 cc. of nutrient 

 gelatine are introduced, and the whole sterilised by steaming for 

 half an hour for three successive days. After the final sterilisation, 

 the cylinder is rotated on its long axis, so that the nutrient medium 

 solidifies in the form of a coating over the whole of the interior. 

 When required for use, the cotton-wool plug is removed from the 

 small glass tube, and the latter connected with the upper flask by 

 means of the india-rubber tubing. 



The apparatus is placed in the air which is to be examined, the 

 outer india-rubber cap removed from the glass cylinder, and the 

 upper flask tilted until the water begins to flow into the lower one. 



