118 ESSAYS ON BACTERIOLOGY. 



cult to separate the different varieties which grow in 

 confusion in the liquid. The invention by Koch' of 

 the solid culture soil has made the separation of micro- 

 organisms rapid and, as a rule, easy. The substances 

 used for this purpose have been many, but experience 

 has singled out a few which answer all ordinary pur- 

 poses. The simplest of these is the surface of a 

 cooked potato, upon which many bacteria may be sat- 

 isfactorily grown. But the potato has been largely 

 superseded by culture soils or media having an al- 

 buminous basis, such as that derived from meat. In 

 order to solidify this meat extract or bouillon, about 

 10 per cent, of gelatin is added, giving a clear, firm 

 product, the beef -gelatin soil. But since gelatin soft- 

 ens at the body or incubator temperature, it has in 

 recent years been improved upon by the use of an 

 Asiatic sea-weed called agar-agar. This substance, of 

 which about 2 per cent, is added to the bouillon, gives 

 us the now widely used beef-agar culture medium, 

 having the gTeat advantage of remaining solid at the 

 heat of the body and of the incubator. It therefore 

 serves for the cultivation of bacteria which require 

 such temperatures for their successful growth. In 

 order to retain as much as possible of the albuminous 

 constituents of the beef after the repeated heating 

 necessary in its preparation, it is either peptonized or 

 artificially digested, or pre-digested albumen in the 

 form of peptone is added. If to this peptonized beef- 

 agar 4 or 5 per cent, of glycerin be added, its field of 



