Thermodynamic Properties of Air. 303 



§ 12. Determination of Temperatures. — All temperatures 

 referred to in this paper are reduced to the scale of the con- 

 stant-volume hydrogen-thermometer. In all experiments at 

 temperatures below zero there was placed in contact with the 

 cooled bulb s, in the same freezing-mixture, the bulb of a 

 hydrogen-thermometer. Yet the temperatures of the bulb 

 during the experiments on expansion were not read directly 

 on the hydrogen- thermometer — because of the waste of time 

 unavoidable in such readings, and because of the slowness 

 of indications, which renders the hydrogen-thermometer un- 

 suitable to follow rapid variations of temperature. For this 

 purpose I constructed a small working thermometer based on 

 the variations of electric resistance of a fine platinum wire. 

 A description of this instrument will be found in the appendix 

 to the present paper. Here it will be sufficient to say that its 

 sensibility was about -^q degr. Centigr. and its quickness very 

 considerable. 



This electric thermometer has been compared very often 

 with the hydrogen-thermometer, and a table has been drawn 

 interpreting its indications in terms of the hydrogen-scale. 

 Nevertheless I never used the electric thermometer otherwise 

 than under control of the hydrogen-thermometer, because 

 slight secular changes of its resistance manifested themselves. 

 Comparisons of the working and the hydrogen- thermometer 

 were made in the intervals between two consecutive experi- 

 ments on expansion. 



During the experiments themselves an assistant read the 

 electric thermometer a first time simultaneously with the 

 charging of the bulbs, a second time immediately after dis- 

 charging them (it will be understood that the freezing-mixtures 

 used to obtain very low temperatures do not keep their tem- 

 perature quite steady) ; the mean of these two readings has 

 been accepted as the temperature (0) of the bulb. Finally, 

 the temperature of the bulb was determined a third time (0') 

 during the measurement of the gas- quantity in the eudio- 

 meters ; this temperature has been used to calculate the exact 

 value of the gas-quantity remaining in the bulb. 



§ 13. The Low Temperatures. — In the manner explained 

 above I executed some hundred and twenty determinations of 

 the coefficient of expansion a Pt0 , using different pressures up 

 to 130 atmospheres. One of the bulbs of the apparatus being 

 kept at + 16°, the other was heated or cooled to the following 

 temperatures: |-100 c (steam), 0° (ice), —35° (a freezing- 

 mixture of pounded ice and crystallized chloride of calcium), 

 — 78°' 5 (solid carbonic acid and ether), — 103°' 5 (liquid 

 ethylene, boiling under atmospheric pressure) , —130°, —135°, 



Y 2 



