124 Manual of Veterinary Microbiology. 



For filtrations of this kind we have employed with 

 advantage a special support bearing a simple glass 

 funnel provided with a lid and adjusted above a con- 

 ical vessel. The gelatinous mass to be filtered hav- 

 ing been introduced into the funnel upon an ordinary 

 filter, the whole is placed in the Koch steam sterilizer. 

 We thus obtain a rapid filtration without desiccation 

 or loss of the fluid, since the filtration takes place in 

 an atmosphere of steam. The agar and gelatin hav- 

 ing been poured into tubes and sterilized, the latter 

 are placed in the cold, some in vertical position, others 

 inclined so as to distribute the mass in a very large 

 oblique layer; either the surface of this layer or the 

 depth, in the vertical tubes, serves as a field for cul- 

 ture. 



Agar has several advantages over gelatin: it is not 

 fluidified by the growth of germs, it remains solid at 

 40°, and, therefore, admits of cultures in the incu- 

 bator at 39°; it may be subjected to cooking for a 

 long time without losing. its gelatinizing power. On 

 the other hand, it has the disadvantage of being al- 

 ways slightly cloudy, is not well adapted for plate 

 cultures, and gives cultures which are not well de- 

 fined. 



Agar-gelatin. — With the aim of making plate cul- 

 tures at the temperature of 39°, Jensen has recom- 

 mended, for the solidification of the bouillon, a mix- 

 ture of agar and gelatin. He adds to the bouillon 5 

 per cent of gelatin and 0.75 per cent of agar. The 

 preparation is very clear, it is liquefiable at a tempera- 

 ture which does not kill the germs, and it remains 

 solid at 39°. 



