AGAR-AGAR MIXTURE. 97 



Dr. Angus Smith has also suggested an excellent method " of 

 measuring the amount of organic life existing in the water by means 

 which may be called purely chemical." He measures their activity 

 by their power of evolving hydrogen during the decomposition of 

 sugar, which they bring about. He proceeds as follows : — " Tubes 

 seven and a quarter inches long and three-quarter inch diameter were 

 filled with the water to be examined, to which one per cent, grape 

 sugar was added. These were inverted, sealed at the bottom with 

 mercury, and allowed to stand. In every case there was a control ex- 

 periment made by using the purest distilled water, to which the same 

 amount of sugar was added." After five days gases commenced to 

 collect, and, in most specimens of water examined, gas was eliminated. 

 He then washed the gas with caustic potash, and, by means of this 

 and pyrogallic acid, found that carbonic acid was present in small 

 quantities only, and that oxygen was absent. The gas was fired with 

 oxygen, and found to be composed almost entirely of hydrogen and 

 nitrogen, and this evolution of hydrogen by the micro-organisms 

 acting on organic matter is made the measure of their vital activity. 



{D.) Cultivation in Sterile Agar-Agar Mixture. 



47. In consequence of the fact already mentioned, that the five per 

 cent, gelatine solution liquefies if its temperature be slightly raised, a 

 more resisting substance has been introduced by Maddox and Koch, 

 which is equal to peptonised jelly in its nutritive qualities and in its 

 transparency, and which will remain solid until a very high temperature 

 (80° C.) is reached. Agar-agar consists of the dried sliced stem of a 

 fucus, and is largely used in cuhnary science under the name of Japan 

 isinglass. If this substance be added in the proportion of one per cent, 

 to the beef-peptone broth used in preparing peptonised jelly, as already 

 described (§ 43, p. 72), and the gelatine be omitted from the mixture, 

 a very nourishing solid medium is produced. Its preparation and 

 sterilisation are completed in precisely the same manner as was 

 described in the case of the jelly, and it is used in the same ways as 

 that substance, over which it has the great advantage in certain cases of 

 being capable of constant incubation, even at high temperatures, with- 

 out losing its solidity. 



