20 MICRO-ORGANISMS IN WATER 



again well shaken to continue the dispersion of the 

 bacteria throughout the culture fluid, which is then 

 poured on to plates, into dishes or flasks, and allowed to 

 solidify, and then kept at a temperature of about 37° C. 

 LofHer^ has found that the addition of other materials 

 in small quantities to the blood-serum, while in no way 

 interfering with its subsequent solidification, increases 

 its value as a culture medium. For this purpose meat 

 extract is recommended to which 1 per cent, peptone, 

 1 per cent, grape sugar, and O'S per cent, common salt 

 have been added ; the whole of this mixture is boiled 

 up, neutralised with carbonate of soda, and then heated 

 on the water-bath until the albuminates have separated 

 out, after which it is filtered and sterilised in the steamer. 

 This broth is then, on cooling to 50°, thoroughly mixed 

 with the blood-serum, and the whole sterilised by dis- 

 continuous heating, and subsequently solidified, as 

 already described. The broth should be added in the 

 proportion of 1 part to every 3 j^arts of serum. 



A number of other variations in the composition of 

 nutritive serum by the addition of different materials 

 may of course be introduced. 



Potato cultures. — The common potato forms one of 

 the most valuable and handy of solid culture materials, 

 inasmuch as the majority of micro-organisms grow 

 luxuriantly upon it, and its preparation is a matter of 

 the greatest simphcity. Again, it acts as a restorative 

 to some bacteria which have become weakened; thus 

 the Bacillus prodigiosus, so well known on account of the 

 magnificent red pigment which it elaborates, after a 

 time entirely loses the power of producing this charac- 

 teristic colour, and gives rise only to a dirty grey white 

 substance ; but if a portion of such a degenerated growth 



^ Mittheilungen aus deni haiserlichen Gesundheitsamte, vol. ii. 1884, 

 p. 452. 



