CULTIVATION WITHOUT OXYGEN. 117 



this group a number of devices are employed for the 

 exclusion of oxygen from the cultures. 



Koch's method. Koch covered the surface of a gelatin 

 plate, which had been previously inoculated, with a thin 

 sheet of sterilized isinglass. The organisms which grew 

 beneath it were supposed to grow without oxygen. 



Hesse's method. Hesse poured sterilized oil upon the 

 surface of a culture made by stabbing into a tube of 

 gelatin. The growth that occurred along the track of 

 the needle was supposed to be anaerobic in nature. 



Method of Buchner. The plan suggested by Buchner 

 of allowing the cultures to develop in an atmosphere 

 robbed of it.s 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 has been deposited 1 gramme of pyrogallic 

 acid and 10 c.c. of -^ normal' caustic jjotash solution. 

 The larger tube is then tightly plugged with a rubber 



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

 grammmes of the dissolved substance as are indicated by its molec- 

 ular equivalent. The equivalent is that amount of a chemical com- 

 pound which possesses the same chemical value as does one atom of 

 hydrogen. For example : One molecule of hydrochloric acid (HCl) 

 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 (H^SO^) 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 grammes to 

 the litre as the number of its molecular weight— 56.1 grammes to 

 the litre of water. 



6* 



