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Determination of Endothermic Gases by Combustion. 



By A. R. Middleton. 



Endothermic gases such as ethylene and acetylene, even when mixed 

 with sufficient air to form an explosive mixture, may be accurately and 

 safely determined by combustion in a gas pipette provided the following 

 conditions are observed : ( 1 ) Presence of a considerable excess of oxygen ; 

 (2) admixture with an exothermic gas; (3) slow admission of the com- 

 bustible gases to the combustion pipette; (4) application of heat from 

 below on the entering combustible gases; (5) reduced pressure. These 

 conditions are secured by using a Winkler-Dennis gas combustion pipette, 

 the platinum spiral being placed as near the juncture of the capillary 

 with the pipette as possible without endangering the glass; mixing the 

 endothermic gases with one to two volumes of pure hydrogen ; and slowly 

 leading this mixture into oxygen instead of the reverse as is usually done 

 in combustion of the methane and hydrogen of illuminating gas. 



The combustion is carried out as follows : The hydrogen used as a 

 diluent is generated in a Hempel hydrogen pipette from zinc free from 

 carbon ; the requisite quantity is drawn into a burette, measured and 

 transferred to a mercury pipette ; a measured volume of acetylene or 

 ethylene is then driven over into the hydrogen and the gases thus mixed 

 drawn back into the burette. About 10 cc. more than the theoretical 

 amount of oxygen required for the combustion is measured and trans- 

 ferred to the combustion pipette. The burette containing the mixed com- 

 bustible gases over mercury is connected with the pipette and the level 

 bulb of the latter so placed that the oxygen in the pipette is under a re- 

 duced pressure of one or two centimeters of mercury. The current is then 

 turned on and the resistance so adjusted that the spiral is maintained at 

 a bright red heat. The pinch-cock on the rubber connection of the burette 

 with the capillary arm of the pipette is opened; the expansion of the 

 oxygen by the heat from the spiral approximately balances the reduced 

 pressure and little or no gas enters the pipette on opening the pinch-cock. 

 The screw pinch-cock on the connecting tube of (lie burette and its level- 

 ing tube is then slightly opened and so adjusted that the flow of gas into 

 the pipette is about 2 cc. per minute. After proper adjustment is effected 



