and Diathermancy of Air and Hydrogen. 405 



I had observed that, if the hot liquid in the brass vessel was 

 allowed to cool freely, the astatic needle nevertheless after 

 some time assumed a maximum deflection, indicating a state 

 of equilibrium between the influx and loss of heat. It is 

 known that tolerably strong deflections of the needle of a 

 highly sensitive galvanometer correspond to but slight altera- 

 tions of temperature at the soldered junction. If, therefore, 

 we know the temperature of the brass vessel (or rather that of 

 its bottom) at the commencement of the experiment, and the 

 temperature of its fluid contents for which the deflection of 

 the needle is greatest, then the thermal equilibrium, upon 

 which the highest temperature at the soldered junction de- 

 pends, must be very nearly proportional to the difference of 

 those two temperatures of the brass vessel, at least so long 

 as the thermal differences are small. 



Accordingly the brass vessel, at the commencement of the 

 experiment, was filled with water at such temperature that the 

 needle pointed to 0° and remained stationary. This having 

 been effected, part of the cold water was taken out and replaced 

 by hot water ; the temperature of the mixture was observed, 

 and simultaneously the time of pouring in was noted. The 

 temperatures were taken by a thermometer whose bulb was in 

 contact with the bottom of the brass vessel. 



The following Table gives the chief results of some of the 

 experiments carried out in this manner. The figures in the 

 column marked T record for each of the stated gases the tem- 

 peratures of the water : — first, at the commencement of the ex- 

 periment ; secondly, after mixing the hot and the cold water ; 

 and. thirdly, at the moment when the deflection of the needle 

 was at its maximum. The differences between the first and 

 third figures are given under t ; whilst r shows the greatest 

 corresponding deflections, whose real and comparable values 

 are placed in brackets underneath them. These values were 

 determined by a method described in a former paper *. I do 

 not venture to ascribe to them a higher importance than that 

 of approximations merely, inasmuch as for deflections of the 

 needle exceeding 40° the differences between the observed and 

 calculated values become far too great to warrant correctness 

 within a few degrees. The figures under z express in minutes 

 the time from the beginning of the experimentuntil the junc- 

 tion reached its maximum temperature. The tension of the 

 gases in the interior of the glass cylinder is given in milli- 

 metres under p ; and, finally, the last column contains compa- 

 rable values expressing the intensity of the thermal action on 

 the junction. 



* Pogg. Ann. Bd. civ. S. 98. 



