TRANSACTIONS OF SECTION A. 623 



ratelv measuring the vapour-pressures of solid and liquid. This i3 particularly well 

 shown by acetic acid : at a temperature of 2-72° the pressures of the vapour in 

 contact with liquid was 4'0 mm., while at 2-86° the pressure of vapour in contact 

 with solid was 2'8 mm. The individual determinations amount to more than a 

 hundred and are perfectly concordant with each other, furnishing two curves which 

 diverge at temperatures below the melting point of the solid acid, 16 - 4°. 



These results were obtained by distilling the substance which was made to coat 

 the bulb of a thermometer under different pressures, and observing the temperatures 

 corresponding to these pressures. 



As in the case of ice and water, the differences of pressure were too minute to 

 estimate with great accuracy, the apparatus was modified, so that ice, covering the 

 bulb of one thermometer, and water adhering to cotton wool wrapped round the 

 bulb of another thermometer, were exposed to the same low pressures, these 

 pressures being variable at will, and the differences of temperature shown by the 

 thermometers were noted. Assuming the formula given by Regnault for the 

 vapour-pressure of liquid water to be correct, the theoretical vapour-pressure of ice 

 was calculated and was fouud to be identical with the authors' experimental results. 

 A series of determinations of the vapour-pressure of ice at very low temperatures 

 was also found to be in accordance with the calculated results. 



Experiments made by the usual method of heating the solid placed over mercury 

 in a barometer-tube gave capricious results, even when care was taken to ensure 

 absolute purity and complete exclusion of air. 



On the Law of Total Radiation at Hi</h Temperatures. 

 By Professor J. Dewar, M.A.,F.R.S. 



5. On Loss of Heat by Radiation and Convection as affected by the dimensions- 

 of the cooling body, and on Cooling in Vacuum. By J. T. Bottojiley, 

 M.A., F.R.S.E. 



In the course of a series of experiments on the heating of conductors by the 

 electric current, which were carried on during the past winter, I obtained a con- 

 siderable number of results which both gave me the means of calculating the 

 Emissicity for heat in absolute measure of various surfaces under different circum- 

 stances, and also caused me to undertake a number of special experiments on the 

 subject. These experiments are still in progress, and I am making preparation for 

 a more extended and complete series ; but a brief notice of some of the results 

 already arrived at may not be without interest to the British Association. 



The experiments were made on wires of various sizes, some of them covered and 

 some of them bare, cooling in air at ordinary temperatures, and at normal and also 

 at very much reduced pressures. 



The mode of experimenting was as follows : — A. current passing through a wire 

 generates heat the amount of which is given by Joule's well-known law : — ■ 



H = C»R/J ; .... (1) 



where C is the current, R the electric resistance, J Joule's equivalent, and H the 

 quantity of heat generated per unit of time ; each being reckoned in 0. Or. S. units. 

 Let I be the length of the wire, d its diameter, and <r t the specific resistance of 

 the material at temperature t° (at which temperature let us suppose that the wire, 

 in the given external conditions, is maintained by the current). Then 



a-,1 4o-J 



R = 



Hence from (1) 



C 3 <w 



hrd* Trcf 2 



Consider, now, that the wire suspended in the air is losing heat by its surface ; 



