Section 4. Thermometers. jj 



of the vapor, it was necessary in this case to cause water to circulate 

 through a jacket surrounding one of the condenser tubes. Pure naph- 

 thalene from Kahlbaum was used to give a temperature of 218, and was 

 found to be an ideal substance. a-Bromnaphthalene was employed for 

 the next higher temperature (281), as it seemed to be the only available 

 substance ; it is not convenient, however, since it decomposes slowly upon 

 boiling with formation of tar and hydrobromic acid (which attacks the 

 outside of the bomb) ; it must therefore be frequently removed from the 

 heater and redistilled. The highest temperature (306) was maintained 

 with benzophenone, which shows no change of boiling-point even after 

 many days of continuous heating. 



4. THERMOMETERS. 



The temperature of the liquid xylene bath was measured with an ordi- 

 nary thermometer reading directly to tenths of a degree, and this was 

 checked from time to time against a standard Tonnelot thermometer. 



For the higher temperatures French mercurial thermometers, made by 

 Alvergniat, with a range of 360 and graduation in degrees, were used. 

 By the use of a little reading telescope these thermometers were read with 

 certainty to 0.1. They were standardized as described in section 8 of this 

 article. The mercury column was always completely immersed in the 

 vapor, and to take a reading the thermometer was quickly raised only 

 enough to render the meniscus visible above the top of the heater. 

 Repeated trials showed that the temperature of the bath throughout the 

 space surrounding the bomb varied less than 0.1, so that the exact posi- 

 tion of the thermometer made no difference. Care had to be taken, how- 

 ever, that the mica shield above the bomb did not come in contact with the 

 thermometer stem, thus allowing the condensed vapor coming from the 

 shield to run down and cool the bulb. It was feared that the vapor con- 

 densing on the upper part of the thermometer itself would have the same 

 effect ; but this was proved not to be the case by fastening a small inverted 

 watch-glass about midway on the thermometer stem ; this carried off the 

 drip from the upper part of the stem, but did not affect the reading. 



5. DETAILS OF THE CONSTRUCTION OF THE BOMB. 



The shell is made of the softest crucible steel obtainable, ductility being 

 desired rather than high tensile strength. Extra weight is not objection- 

 able here, as it would be in the case of a calorimetric bomb ; moreover, 

 fear was entertained that a high-carbon steel might be weakened by the 

 repeated heating and cooling to which the bomb was to be subjected. The 



