THE PUMP 327 



that the oxid in immediate contact with the organic matter be in 

 a sufficiently fine state of subdivision, and that the substance 

 itself be very finely powdered and intimately mixed with the 

 oxidizing material. Failure to attend to these precautions will 

 bo followed by an incomplete combustion and a consequent deficit 

 in the volume of nitrogen obtained. 



The copper oxid before using is ignited, and is best filled into 

 the tube while still warm by means of a long pointed metal scoop, 

 or other convenient method. The copper spiral, after use, is re- 

 duced at a red heat in a current of hydrogen, and may thus be 

 used many times. 



287. The Pump. Any form of mercury pump which will 

 secure a complete vacuum may be used. A most excellent one 

 can be arranged in any laboratory at a very small expense. The 

 pump used in the laboratory of the Bureau of Chemistry for 

 many years answers every purpose, and costs practically nothing, 

 being made out of old material not very valuable for other use. 



The construction of the pump and its use in connection with 

 the combustion tube will be clearly understood from the follow- 

 ing description : 



A glass bulb I is attached, by means of a heavy rubber tube 

 carrying a screw clamp, to the glass tube A, having heavy walls 

 and a small internal diameter, and being one meter or more in 

 length. The tube A is continu d in the form of a U, the two 

 arms being joined by very heavy rubber tubing securely wired. 

 The ends of the glass tubes in the rubber should be bent so that 

 they come near together and form the bend of the U, the rubber 

 simply holding them in place. This is better than to have the 

 tube continuous, avoiding danger of breaking. A three way tube, 

 T, made of the same kind of glass as A, is connected by one arm, 

 a, with the manometer B, by a heavy rubber union well wired. 

 The union is made perfectly air-tight by a tube filled with mercury 

 held in place by a rubber stopper. The middle arm of the tee, 

 a', is expanded into a bulb, E, branching into two arms, one of 

 which is connected with A and the other with the delivery tube 

 F, by the mercury-rubber unions, MM', just described. The in- 



