BACTERIA IN THE SOIL 1 39 



is weighed on the balance, and then added to 1000 cc. of 

 distilled sterilised water in a sterilised flask, in which it is 

 thoroughly mixed and washed. From either of these two 

 different sources it is now possible to make sub-cultures and 

 plate cultures. The procedure is, of course, that described 

 under the examination of water (p. 41 et seq.), and Petri's 

 dishes, Koch's plates, or Esmarch's roll cultures are used. 

 Many of the commoner bacteria in soil will thus be detected 

 and cultivated. But it is obvious that this by no means 

 covers the required ground. It will be necessary for us here 

 to consider the methods generally adopted for growing an- 

 aerobic bacteria, that is to say those species which will not 

 grow in the presence of oxygen. This anaerobic difficulty 

 may be overcome in a variety of ways. 



1. The air contained in the culture tube may be removed 

 by ebullition and rapid cooling. And whilst this may ac- 

 curately produce a vacuum, it is far from easy to introduce 

 the virus without also reintroducing oxygen. 



2. The oxygen may be displaced by some other gas, and 

 though coal-gas, nitrogen, and carbon dioxide may all be 

 used ^for this purpose, it has become the almost universal 

 practice to grow anaerobes in hydrogen. The production of 

 the hydrogen is readily obtained by Kipp's or some other 

 suitable apparatus for the generation of hydrogen from zinc 

 and sulphuric acid. The free gas is passed through various 

 wash-bottles to purify it of any contaminations. Lead 

 acetate (i-io per cent, water) removes any traces of sul- 

 phuretted hydrogen, silver nitrate (i-io) doing the same for 

 arseniated hydrogen ; whilst a flask of pyrogallic acid will 

 remove any oxygen. It is not always necessary to have 

 these three purifiers if the zinc used in the Kipp's apparatus is 

 pure. Occasionally a fourth flask is added of distilled water, 

 and this or a dry cotton wool pledget in the exit tube will 

 ensure germ-free gas. From the further end of the exit 

 tube of the Kipp's apparatus an india-rubber tube will carry 



