LABOBATOBY BESEAECH, ETC. 267 



The tubes, which have two reservoirs and two 

 tapered ends (Figs. 105, 106), are very numerous in 

 Pasteur's laboratory. They are ranged on a stand in 

 the way shown in the figure. 



It is ascertained that the contents of the tubes are 

 really sterilized by leaving them for several days in 

 a stove which i^ maintained at a temperatur(3 of 35°. 



In addition to the culture liquids already indi- 

 cated, many others consist of various solutions of 

 phosphates of lime and potassium, albuminous solu- 

 tions, etc. 



Solid Nutritive Svhstances. — In order to isolate the 

 different species of bacteria, and to obtain pure cul- 

 tures, solid substances ai-e now preferred : eggs, slices 

 of potatoes and carrots, but especially gelatine and 

 gelose — which comes from Japan ready for use, and is 

 said to be extracted from a marine alga — and the gela- 

 tinized serum of the blood of oxen. All these sub- 

 stances are transparent, so that the cultures can be 

 easily observed in glass tubes. Koch, in his Berlin 

 laboratory, makes almost exclusive use of solid media, 

 which are first sterilized by similar precautions. 



In order to obtain pure cultures, all kinds of germs 

 are first allowed to grow ; then a very small amount 

 of them is taken from the culture medium, and 

 transferred to the sterilized medium, in which fewer 

 microbes naturally appear. After several repetitions 

 of this transplantation, sufficiently pure cultures may 

 generally be obtained within a short time. 



