CULTURE MEDIA. 43 
to cool to about 50° C., and an egg, previously broken into one 
hundred grammes of water, is gradually added while stirring the 
liquid with a glass rod. A whole egg is used for a litre of the solu- 
tion. Heat is again applied and the solution is kept at the boiling 
point for about ten minutes, during which time the egg albumen is 
precipitated and carries down with it all insoluble particles, which 
without this clarifying process would have interfered with the trans- 
parency of the medium, even when carefully filtered. The hot 
solution is then filtered. A hot-water funnel (Fig. 17) is usually 
employed, as the gelatin solution does not pass through filtering 
paper very rapidly, and when cooled to near the point of solidifying 
ceases to pass. ; 
The advantages of the gelatin medium are that it is perfectly 
transparent, that it is easily melted for making “‘ plates,” and that 
many bacteria exhibit in it special characters of growth by which they 
may be differentiated from others which resemble them in form. 
The principal disadvantage is the low melting point, which prevents 
us from making use of this medium for cultivating bacteria in an in- 
cubating oven at a higher temperature than about 22° C. for ten-per- 
cent gelatin. 
This disadvantage is overcome by using agar-agar instead of 
gelatin. This is prepared in Japan and other Eastern countries 
from certain species of gelatinous alge. It comes to usin the form 
of bundles of dried strips, which form a stiff jelly when dissolved in 
water in the proportion of one to two per cent. This jelly remains 
solid at a temperature of 40° C. and above. It was first employed 
by Hesse, one of Koch’s collaborators in the office of the imperial 
board of health of Berlin. Koch, who was in search of a trans- 
parent jelly which would stand the temperature required for the cul- 
tivation of certain pathogenic bacteria (37° to 38° C.), quickly recog- 
nized its value and introduced it into general use. 
The agar-agar jelly is more difficult to filter than the gelatin 
medium, and some skill is required in order to obtain a transparent 
solution. It will bear long boiling without losing its quality of 
forming a stiff jelly. From ten to twenty grammes are added toa 
litre of flesh infusion, or we may make a peptonized agar in accor- 
dance with the following formula which is given by Salomonson : 
Add to one litre of distilled water five grammes Liebig’s extract, 
thirty grammes peptone, five grammes cane sugar, fifteen grammes 
agar. Cook for an hour, render slightly alkaline, and cool to below 
60° CG. Clarify and cook again for an hour or more. 
Glycerin-agar is made by adding five per cent of glycerin to 
the peptonized agar made by the above formula or by the use of the 
flesh-peptone infusion. This is a very favorable medium for the cul- 
tivation of the tubercle bacillus—first used by Roux and Nocard. 
