BACTERIA IN RELATION TO PLANT DISEASES. 



hydrogen or arseniureted hydrogen ; it should therefore be passed not too rapidly 

 through the following solutions in the order indicated : Saturated solution of lead 

 acetate, 5 per cent solution of silver nitrate, 10 per cent potassium permanganate, 

 i o. per cent sodium hydrate containing pyrogallol, distilled water. When ready 

 for use the purity of the hydrogen may be tested by burning in test-tubes (mouth 



down , and also, if necessary, by the ordinary 

 methods of gas-analysis. To avoid the evolu- 

 tion of hydrogen sulphide the generator may be 

 plunged into a jar of ice-water, as shown in 

 plate 7. Special care must be taken in sealing 

 jars containing hydrogen, otherwise it will 

 escape. In use, the gas is allowed to bubble 

 slowly through the fluids into the culture- 

 chamber, a large well-clamped Novy jar, the 

 other tubular opening of which is connected 

 air-tight with the tube of the vacuum pipe. 

 The jar is first pumped out and the hydrogen 

 is then allowed to enter. When the jar is full, 

 the glass stopcock nearest it (at the left in plate 

 7) is turned, and then, after allowing a few 

 minutes for diffusion, the mixture of air and 

 gas is pumped out. The vacuum cock is then 

 turned off and the hydrogen is again turned 

 on slowly. This process is repeated five or 

 six times, the gas being passed into the jar 

 very slowly the last two times, so that it may 

 be washed very clean. The Novy jar is then 

 Fig. 52.* sealed, disconnected, and set away in the dark. 



The gas must,, of course, enter each wash-bottle through the long stem. It is 

 desirable to have each wash-bottle two-thirds full of fluid, and there must be no leaks 

 in any part of the apparatus.f The hydrogen should be cut off before each exhaustion 

 of the jar. by turning the stop-cock nearest the jar. The cock also should be turned 

 off before sealing glass tubes with flame and it must, of course, be known that the 

 gas is free from admixture with air, otherwise an explosion will occur. 



It is easier to keep air out of gases than to remove it. The greatest care 

 should therefore be taken to drive it out of a culture medium before it is inocu- 

 lated. For the same reason gas should be allowed to flow for some time before 

 it is collected- so as to displace air which may have diffused into the generator and 

 wash-bottles. This is also the reason why the water which is used to dilute the acid 

 and the marble chips should be boiled. If there is much air mixed with the gas 

 .t is riot at all likely that a single wash-bottle of sodium hydroxide and pyrogallol, 



*FiG. 52. Hempel's simple pipette for liquid reagents used in gas-analysis. Breadth of stand, 

 7 inches. 



t Consult a paper by Ewell, Centralb. f. Bakt., 2 Abt, III Bd., p. 188. 



