138 COLLOIDS IN BIOLOGY AND MEDICINE 



assumed. Thus, for instance, the melting point of gelatin is raised 

 by grape sugar and glycerin, whereas that of agar is reduced. NaCl 

 elevates the melting point of agar and depresses that of gelatin (H. 

 BECHHOLD and J. ZiEGLER* 2 ). 



Agar has a very strong tendency to gelatinize; even 1 gm. per 

 liter gelatinizes at 0. This great gelatinizing capacity led ROBERT 

 KOCH to make his culture media of agar, and permitted him to grow 

 cultures of bacteria on solid media at body temperature. Gelatin 

 media which had been used at first melt at 37 C., and could, ac- 

 cordingly, only be used at room temperature. 



Electrolytes as well as nonelectrolytes alter the gelatinization 

 time of agar. Nitrates, iodids, sulphocyanids, benzoates, urea and 

 thiourea lengthen it; chlorids, bromids, acetates and salts of poly- 

 basic acids shorten it. 



Cellulose is for plants what bones are for animals. It forms the 

 framework which maintains their shape. If it is to fulfill this function 

 it must be insensitive to the chemical influences of the plant juices, 

 and must not be able to swell. Wooden relics are by no means un- 

 common; only in exceptional instances are fats and proteins or 

 gelatinous constituents seen after thousands of years, and then only 

 under very unusually favorable conditions, as in the desert climate 

 of Egypt. Wood, even uncarbonized, is a common relic not only for 

 Egyptian archaeologists and travellers in the Turanian deserts, but 

 it has frequently been preserved in our own climate and even in 

 water. Oak bridge piles dating from Roman times have been found 

 in the Rhine, wood carvings and wooden buckets in the springs of 

 Salzburg, fragments of boats of the lake dwellers, in the Swiss lakes, 

 and those of the Vikings in the peat bogs of North Germany and 

 Jutland. Stability of form, in other words, a slight swelling capacity, 

 makes wood, next to stone, metal and bone, suitable for many pur- 

 poses. 



Cellulose, the principal constituent of wood, is extremely inactive 

 and is only split up into soluble sugars (chiefly grape sugar) by 

 strong chemical action (acids concentrated or under pressure), or 

 by specific ferments (bacteria in the intestines of ruminants). 



Cellulose not only has a high adsorptive capacity for dyestuffs, 

 but even true suspensions are fixed at its surface. For this reason 

 cellulose has recently been used like charcoal as a clarifier and as a 

 filter for turbid liquids. 



