222 BACTERIOLOGY. 



layers of the medium. In his other method he employs 

 a special tube, known as " the Liborius tube." Its con- 

 struction is shown in Fig. 39. 



Through the side tube hydrogen is passed until it re- 

 places all the air ; the contracted parts, both of the neck 

 of the tube and the side arm, are then sealed in the 

 flame. 1 This tube can be used for either solid or liquid 

 media, but, owing to its usual small capacity, gives 

 better results with fluid media. (For precautions in 

 using hydrogen, see note to FrankePs method, page 

 223.) 



Method of Buchner. The plan suggested by Buchner, 

 of allowing the cultures to develop in an atmosphere 

 robbed of its oxygen by pyrogallic acid, gives very good 

 results. In this method the culture, which is either a 

 slant- or stab-culture in a test-tube, is placed tube, 

 cotton plug, and all into a larger tube, in the bottom 

 of which have been deposited 1 gramme of pyrogallic 

 acid and 10 c.c. of -fa normal 2 caustic-potash solution. 



1 As the tubes come from the maker the contracted parts marked x 

 in the cut are usually so thick as to render the sealing in the flame 

 during the passage of hydrogen somewhat troublesome ; it is better 

 to draw them out in the flame quite thin before passing the hydrogen 

 into the tube. This makes the final sealing a matter of no difficulty. 



2 A normal solution is one that contains in a litre as many grammes 

 of the dissolved substance as are indicated by its molecular equivalent. 

 The equivalent is that amount of a chemical compound which possesses 

 the same chemical value as does one atom of hydrogen. For example : 

 one molecule of hydrochloric acid (HC1) has a molecular weight and 

 also an equivalent weight of 36.5 ; a molecule of this acid has the 

 same chemical value as one atom of hydrogen. Its normal solution is 

 therefore 36.5 grammes to the litre. On the other hand, sulphuric acid 

 (HzSOi) contains in each molecule two replaceable hydrogen atoms ; 

 its normal solution is not, therefore, 80 grammes (its molecular weight) 

 to the litre, but that amount which would be equivalent chemically to 

 one hydrogen atom, viz., 40 grammes (one-half its molecular weight) 

 to the litre. A normal solution of caustic potash contains as many 



