30 BACTERIA IN RELATION TO PLANT DISEASES. 
parasites is restrained by some nutrient gelatins which are neutral or only feebly 
alkaline to litmus, it is advisable to add to a part of the stock more caustic soda than 
is commonly used, 2. é., enough to render it neutral to phenolphthalein (strongly 
alkaline to neutral litmus), especially if gelatin is selected as the first medium for 
isolation experiments ; otherwise perplexing failures may result. 
(c) Streaks.—Record the character of the streak, whether wet or dry, smooth, 
wrinkled, or rough, thin or piled up, margin well defined or indistinct. Note also 
whether the surface is ever iridescent, whether growths are sent down from the under 
surface into the substratum, whether the streak spreads rapidly and widely over the 
surface or very slowly. ‘The sur- 
face behavior depends to some 
extent on the motility of the 
organism, on the amount of water 
in the surface layers, z. e., whether 
the slants are fresh or old, and on 
the amount of gelatin in the me- 
dium, which in temperate climates 
should usually be 10 per cent, but 
may be 15 or even 20 per cent. 
By minimizing heat in prepara- 
tion and by increasing the quan- 
tity of gelatin to 20 or 30 per 
cent a medium may be obtained 
which will remain solid at 30° C. 
Growth is less satisfactory, how- 
ever, on such a dense medium, 
or at least was in the few tests 
PHOTOGRAPHIC GELATINE made by the writer. Chester has 
mencrany” applied the ordinary botanical 
ee terminology to the varying mar- 
gins of colonies, etc.,and has pub- 
lished some useful figures (’or, 
Bibliog., III). 
No substance used in the bac- 
Fig. 28.* teriological laboratory is so uncer- 
tain and variable in its composition as gelatin. ‘The gelatin from different factories 
varies greatly and hardly any two batches from the same factory are alike. One glue 
chemist has defined gelatin as “80 per cent glue, 10 per cent dirt, and 10 per cent 
doubt.” It varies greatly in its melting point and power of setting, and in amount 
of peptones and albuimoses it may contain, which is sometimes large. It always con- 
tains calcium salts and phosphates, which are often antiseptic, and the nature of which 
varies according as hydrochloric or sulphurous acid has been used in its manufacture. 
Formaldehyde is sometimes added to it, we are told ; and occasionally agar also, it is 
UST RE CEPT wr A ORY hace 
*Fic. 28.—Nelson’s photographic gelatin No. 1. Recommended for bacteriological use. 
