34 THE STORY OF THE BACTERIA. 



from the surface the exact mode of growth of 

 the particular form of bacteria introduced into 

 the tube, and thus we learn a new set of char- 

 acteristics for the different species. 



But if we need to keep our bacteria at a 

 higher temperature than that of an ordinary 

 room, say at the temperature of the body, at 

 which alone some forms will grow, the gelatin 

 would melt and the bacteria would be scat- 

 tered through it, and the characteristic mode 

 of growth of the masses or colonies would be 

 lost. So, for this purpose we use, instead of 

 gelatin, Agar-Agar, a material derived from a 

 sea-weed, which in one per cent, solution forms 

 a gelatinous solid transparent mass, which 

 may be heated to above the temperature of 

 the body without fluidifying. To this are 

 added, as to the gelatin, beef-tea, pepton, etc. 



By the use of these various soils, or "cult- 

 ure-media," as they are called, we can arrive 

 at a series of characteristics in the mode of 

 growth of various bacteria by which, together 

 with their form when seen under the micro- 

 scope, we can distinguish them one from the 

 other, just as the naturalist distinguishes from 

 each other nearly related animals and plants. 



