TRANSACTIONS OF SECTION A. 565 



14. Report of the Committee on Underground Temperature. 

 See Reports, p. 107. 



15. Report of the Ben Nevis Committee.— ^ee Reports, p. 108. 



16. On Recent Researches in the Infra-Red Spectrum. 

 Bxj Dr. S. P. Langley. 



This paper was ordered by the General Committee to be printed in 

 extenso. — See Reports, p. 465. 



17. .4 nev: Determination of the Ratio of the Specific Heats of certain 

 Gases. By O. Lummer a^id E. Pringsiieim.' 



When a perfect gas expands adiabatically from the pressure p^ to the pressure 

 p^, while its absohite temperature decreases from T, to T., we have 



.- ■°^(£) 



where T means the ratio of the two specific heats. Therefore T can he deter- 

 mined by four corresponding vahies oi p^, p.,,'S^, and T.^. For this purpose we 

 experimented in the following way. 



A copper balloon, nearly globular, containing about 90 litres was placed in a 

 bath of water whose temperature was maintained constant within 0°-01 C. and 

 could be measured by a thermometer. Let this temperature be Tj. The balloon 

 "was iilled with a gas, well dried and pure, which was compressed to the pressure 

 p^, measured by a manometer of sulphuric acid communicating with the balloon. 

 Then the gas was allowed to escape into the atuiospliere through an aperture of 

 the balloon. So it expanded to the atmospheric pressure />., given by the 

 barometer. In this way the quantities ^»,,^;^, and Tj can be found easily. The 

 only difficulty is to determine the temperature T., of the gas at the moment 

 when the expansion is finished and the pressure has attained the value ^^. For 

 this purpose we need a thermometer showing the variable temperature of the gas 

 instantaneously, that is, a thermometer of a negligible small mass. 



A thermometer of the required qualities was formed by a strip of platinum of 

 •an extremely small thickness, which is soldered at both ends to two copper wires 

 insulated from each other and introduced air-tight into the balloon. From these 

 the strip hangs down freely in the middle of the balloon. The strip of platinum 

 with its conducting wires formed one arm of a Wheatstone bridge, so that we were 

 able to measure its electrical resistance, and hence its temperature at every 

 moment. The strip was prepared by the method first used for wires by WoUaston, and 

 adopted for strips by Lummer and Kurlbaum. It was cut out of a platinum silver 

 plate composed of a platinum plate with the thickness 0'6/i and a silver plate (3^1. 

 thick. The middle part of the strip had a length of 10 cm. and a breadth of 

 about 0'2 mm., while the two ends were formed by conducting laps 1-5 cm. long 

 and about 4 mm. broad. The silver was removed by nitric acid only in the middle 

 narrow part, so that the resistance of the conducting laps perfectly disappeared in 

 comparison with that of the middle part. But also these conducting laps are thin 

 enough to take at a short distance from the ends the temperature of the surround- 



' The original source of publication is the Smithsonian Institution in Washington, 

 which kindly granted liberal assistance from the Hodgkins Fund for carrying on 

 this investigation. 



