EE8EARCHES UPON THE CHEMICAL PROPERTIES OP GASES. 



155 



point D, and of the two thermometers E was kept a few degrees higher than F, the 

 purpose being to have the gas heated nearly to the temperature of the hottest part 

 of the oven before it reached that point. Thus it was not possible for the gas stream 

 to exei"t a cooling effect upon the metal-coated asbestos. This ajjparatus is superior 

 to an ordinary sheet-iron oven, as the glass tubes are heated by actual contact rather 

 than by radiation. Repeated trials have shown that if a thermometer be inserted in 

 the side neck, and a second one in one of the long, horizontal iron tubes, the differ- 

 ence in the indications of the two thermometers will amount only to an insignificant 



B 



„{5.-. 



D ;; 



Iron oven used in experiments upon oxidation temperatures. 



fraction of a degi'ee Centigrade. I^^itrogen-fillcd mercury thermometers were used 

 up to 300° in the following experiments, and in order to measure higher tempei'a- 

 tures, metallic salts of known fusing point were employed. A few grains or crystals 

 of the carefully purified salt were placed in a fine glass tube previously drawn out 

 to a point. Such a tube was placed in the side neck of the iron oven and plunged 

 into the iron turnings. The temperature of oxidation of the hydrocarbon gas could 

 then be ascertained to occur between the melting points of two salts. This method, 

 which is simply an adaptation of the process commonly used for the determination 

 of melting points, proved very satisfactory. It was not possible to employ an air 

 thermometer, as this would have necessitated an inconveniently large oven. The 

 following list of substances, with their melting points, includes those which were 

 used in the experiments detailed below. 



Potassium nitrate 33JP 



Potassium chlorate 359° 



Lead iodide 383° 



Cadmium iodide 404'^ 



Barium chlorate 414^ 



