168 PROCEEDINGS OF THE AMERICAN ACADEMY. 



joined by means of brass connectors to the smaller copper wires, L x , L 2 > 

 L 3 , coming from the bomb. A double-throw switch served to connect 

 the conductivity apparatus with 1^ and L 2 or with L : and L 3 . 



3. The Heaters. — Conductivity 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 con- 

 tained 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 temper- 

 ature was 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 ex- 

 pansion 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 onto 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, with a bottom piece welded in. Near the top two pieces of iron 

 pipe, C, about 2 cm. in diameter and 25 cm. long are screwed in, to serve 

 as condensers. These condenser tubes are given a slight pitch, but 

 their outer ends should not be higher than the top of the heater. To in- 

 crease their efficiency, a loose roll of iron wire gauze is put into each of 

 them. The top of the bath, which should be turned off square in the 

 lathe, is covered with a large watch glass, D, in which holes are drilled 

 for the thermometer, T, and the lead-wires to the bomb. A tube of thin 

 sheet iron, Q, about 12 cm. in diameter, with a flauge at the bottom, is 

 placed in the heater and held in the middle by projecting pins. Small 

 holes are drilled through this tube at the bottom, and two rows of large 



