How Bacteria Are Studied 35 



introduced into the gelatin, the cotton plug 

 being removed for an instant for this purpose. 

 Being transparent, the gelatin permits us to 

 see from the sides as well as 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 characteristics 

 for the different species (see Plate I). 



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 i per cent, solution 

 forms a gelatinous solid transparent mass, 

 which may be heated to above the temper- 

 ature 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 "culture 

 media," as they are called, and many others 



