TRANSACTIONS OF SECTION A. 



623 



1807, and 

 ison, B.A., 

 s valuable 



rati'lvnieasuriiif,' the vapour-pressures of solid niid liqukl. Tliis is ]mrticularly well 

 .shown by acetic acid: at a temperature of 2-7'2" the pressures of the vapour in 

 contact with liquid was 4'0 imn., while at 'J-f^(r the pressure of vapour in contact 

 witii solid was i''8 nnii. Tiio individual determinations amoiuit to more than a 

 Imiidred and are perfectly concordant with each otiier, furnishinp: two curves which 

 diviM'fre at temperatures below tlie nieltini,' point of tin; solid acid, 1(5-4°. 



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

 tlie bulb of a thermometer under diti'orent pressures, and observing' the temperatures 

 CDrrcspoudiufr to these pressures. 



As in tilt! case of ice and water, the differences of ]n'essure wtn-e too minute to 

 e.<tiniate with jrreat accuracy, the apparatus was modified, so tliat ice, covering: the 

 jmlh of one ihemiometer, and water adhering to cotton wool wrapped round the 

 Lull) of another thermometer, were exposed to the same low pressures, thest; 

 lire.'^suros being variable at will, and the ditlerences of temjierature shown by the 

 tlierniometers were noted. Assuming the formula given by Itegnault for the 

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

 was calculated and was found to be identical with the authors' experimental results, 

 A series of determinations of tlie 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 

 ill a barometer-tube gave capricious results, even Avhen care was taken to ensure 

 absolute purity and complete exclusion of air. 



4. On the Law of Total Ihvliatlon at Hi</h Temjjeratiires. 

 By Professor J. Dewar, M.A.,'F.L'.S. 



•). 0)1 Loss of Heat by Radiation and Convedioii as affided hy the dimensions 

 of the cooUnrj body, ami on Cooling in Vacuum. By J. T. Bottomley, 

 M.A., F.B.S.E. 



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

 electric current, wulr^i were carried on during the past winter, I obtained a con- 

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

 Em'mi'iti/ for heat in absolute measure of various surfaces under diH'erent cireuni- 

 stanci'S, and also caused mo to undei-take 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 ol them covered and 

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

 iit Acry much reduced pressures. 



Tlie mode of experimenting was as follows : — .Y current passing through a wire 

 ;.'onerates heat the amount of which is given by Joule's well-known law : — 



n -C-R/J ;....(!) 



where C is the current, li the electric resistance. J Joule's equivalent, ami II the 

 iiuaiitity of heat generated per unit of time ; each being reckoned in C. (1. S. units, 

 i^'t / be the length of the wire, d its diameter, and o-, the specific resistance of 

 the material at temperature t° (at which temperatuie h.'t us suppose that the wire, 

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



,, ^ o-y _4fr/ 



{nil' Tid- 

 Hence from (1 ) 



C- 4.r,/ 



J" • .. • t • • 



IL 



(2) 



t'ousider, now, that the Avire suspended in the air is lo'^ing heat by its surnice 



