MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 23 



of 1 litres, are made of tinned copper, are cylindrical in shape 

 and conical at the top ; at the apex of the cone a twice-bent 

 tube (c d) with an enlargement (e) is either soldered or screwed. 

 At one side of the cone is the inoculating tube and glass 

 stopper (a), and at the bottom of the vessel is another tube (5) 

 for drawing off the fermented liquid and the yeast. This tube 

 is provided with a pinch-cock. "When the liquid is sterilised, 

 the bent tube is closed with an asbestos or cotton-wool filter, 

 which is placed over the end (d). 



In the new model (Fig. 8) the bent tube is ground into the 

 upper part of the flask and held fast by means of a screw, 

 allowing the whole of this part to be detached, when the flask 

 is to be cleaned ; the filter is also fixed with a screw to the 

 point of the bent tube. 



For further particulars respecting the treatment of these 

 vessels, see HANSEN'S " Practical Studies in Fermentation." 



6. NUTRITIVE SUBSTRATA. 



With regard to the nutritive substrata, the problem 

 naturally consists in finding those which are best suited to 

 the respective organisms. If they also possess the advantage 

 of being per se less favourable for the development of compet- 

 ing forms, it is a great point gained. The fact must of course 

 be borne in mind, when comparative investigations are made in 

 different directions, that the nutritive liquid must always 

 remain the same. For the investigation of alcoholic ferments 

 HANSEN generally uses hopped wort from the filter-bags ; in 

 special cases of investigations of this kind yeast- water with an 

 addition of glucose, or a solution of cane-sugar or some other 

 sugar, is employed. If it is desired to use a solid nutritive 

 material, the liquid may be mixed with 5 to 10 per cent, of 

 gelatine. Similar liquids, or more frequently, meat-extract 

 with an addition of peptone, are employed for bacteriological 

 investigations ; this mixture is neutralised with sodium car- 

 bonate. Gelatine is used for rendering the medium solid. In 

 case cultures are to be made at temperatures above 30C., 

 agar a jelly derived from sea-algae is used in the proportion 

 of 1 to 2 per cent. For cultures of thermophilous bacteria, 



