630 REPORT— 1895. 



In Table II.— 



L is the heat of evaporation ; 



t the temperature of the boiling-point ; 



M the molecular weight ; 



ML 



-=- the quotient of the molecular heat of evaporation by the absolute 



temperature : according to Tronton's Law this should be constant. 



If the heat of vaporisation of any one liquid be known, the absolute value of 

 the heat of vaporisation for any other liquid can be calculated from the ratio. 

 Water was originally selected as the standard liquid, but it proved to be quite 

 unsuitable : experiments in which one of the liquids was water never gave 

 concordant results. It appeara impossible by any ordinary means to get water so 

 pure that conduction across it and the consequent polarisation may be disregarded. 

 The liquid finally adopted as the standard is benzene, for which L = 94-4 at 80°-2.i 



The heat of evaporation may also be calculated from the thermodynamic 



equation, L = («' — «) JP- • _^, when the necessary data can be found. 



The value of J adopted in these calculations is that given by Griffiths - and 

 used in working out the experiments on benzene. Even if not correct, it is still 

 the right value to use here, because it was determined by means of the same 

 standards as those by which the quantity of heat developed in the benzene experi- 

 ments was determined, so that any errors would eliminate. 



The other data were experimental. 



The agreement between the found and calculated values of L is fairly close in 

 most of the cases examined. The work is still in progress. 



5. On a Harmonic Analyser. By G. U. Yule. 



The author exhibited the instrument, which is fully described in the ' Philo- 

 sophical Magazine ' for April 1895. 



TUESDAY, SEPTEMBER 17. 



The following Papers and Reports were read : — 



1. On the Electrification and Diselectrification of Air and other Gases. 

 By Lord Kelvin, Magnus Maclean, and Alexander Galt. 



§ 1. Experiments were made for the purpose of finding an approximation to 

 the amount of electrification communicated to air hj one or more electrified 

 needle points. The apparatus consisted of a metallic can 48 cm. high and 

 SI cm. in diameter, supported by paraffine blocks, and connected to one pair of 

 quadrants of a quadrant electrometer. It had a hole at the top to admit the 

 electrifying wire, which was 5'81 metres long, hanging vertically within a metallic 

 guard tube. This guard tube was always metallically connected to the other pair 

 of quadrants of the electrometer and to its case, and to a metallic screen surround- 

 ing it. This prevented any external influences from sensibly aflecting the electro- 

 meter, such as the working of the electric machine which stood on a shelf 5 metres 

 above it. 



§ 2. The experiment is conducted as follows : — One terminal of an electric 

 machine is connected with the guard tube and the other with the electrifying wire 

 which is let down so that the needle is in the centre of the can. The can is 

 temporarily connected to the case of the electrometer. The electric machine is 



' Griffiths and Marshall. = PJdl. Trans., 18f A (1893). 



