Sections 2 and j. The Conductivity Apparatus. u 



2. THE CONDUCTIVITY MEASURING APPARATUS. 



The conductance was measured by the ordinary Kohlrausch-Wheat- 

 stone bridge method, using the induction coil and telephone. The slide 

 wire was of platinum-iridium ; it was 1 meter in length and 0.4 mm. in 

 diameter. The resistance coils, 2,000 ohms in all (or 4,000 ohms in a few 

 measurements), were of manganine. The whole conductivity apparatus 

 was mounted on a small portable table so that it could be moved about as 

 the bomb was changed from one heating bath to another. It was always 

 kept at a distance from the heaters. No temperature correction needed 

 to be applied to the resistance coils. Heavy flexible copper leads were 

 used up to within a few centimeters of the top of the heaters, where they 

 were joined by means of brass connectors to the smaller copper wires, 

 L lt L 2 , L 3 , coming from the bomb. A double-throw switch served to con- 

 nect the conductivity apparatus with L x and L 2 or with L and L 3 . 



3. THE HEATERS. 



Conductance measurements were made at about 26, 140, 218, 281, 

 and 306. The first of these temperatures was attained by immersing the 

 bomb in a bath of commercial xylene contained in a double-walled, well- 

 jacketed, metal cylinder. This substance has the advantages that it is a 

 good insulator, non-corrosive, and not very volatile, and that the bomb can 

 be transferred from it directly, without cleaning, into the xylene-vapor 

 bath by which the next higher temperature is attained. The liquid was 

 stirred by a small propeller, and was heated electrically at will with the 

 help of a platinum helix immersed in it. 



For all the higher temperatures, vapor baths were employed, as these 

 furnish the only safe and rapid method of heating. The temperature 

 adjusts itself automatically, and can never rise much above the ordinary 

 boiling-point, thus giving protection against overheating and undue expan- 

 sion of the liquid within the bomb, which by completely filling it might 

 cause it to burst. Moreover, if the bomb should spring a leak, it would 

 be dangerous in the case of a liquid bath ; for the steam, escaping under 

 such pressure, might throw some of the hot liquid upon the observer. 

 Steam leaking out into the hot vapor, on the other hand, causes no annoy- 

 ance further than that arising from the odor of the vapor and the loss of 

 the material in the case of the expensive substances. An air bath would, 

 of course, not be open to this objection ; but the heating would be ex- 

 tremely slow and non-automatic. 



An elevation of one of the heaters all of which were substantially 

 alike with the bomb in place is presented in fig. 2. The bath is made 

 of a piece of wrought-iron pipe A, 16 cm. in diameter and 40 cm. long, 



