20 Mr. E. H. Griffiths and Miss Marshall on the 



These changes in Q s probably indicate the changes in the 

 viscosity of the oil. 



14. Alterations in the Apparatus. 



No alterations have been made except in exterior portions 

 of the apparatus. 



When working with water in 1894 there were, in the tubes 

 leading to the air-pumps, one or two rubber joints which are 

 now replaced by glass ones. 



To prevent condensation, the benzene vapour after issuing 

 from the tank passed over a row of small gas-jets and then 

 down into a small Wolff's bottle, connected with the mano- 

 meter and containing pumice-stone and sulphuric acid. It 

 then passed through another tap * into a large globe (capacity 

 about 35 litres) also containing pumice-stone and sulphuric 

 acid. By means of a water-pump the pressure in this globe 

 was reduced to that of the aqueous vapour at the temperature 

 of the tap-water. Its capacity was so great that the pressure 

 in the Wolff's bottle could, at any time, be brought below 

 that required, at experiments above 20°, by simply opening 

 the tap communicating with the globe. Thus the water- 

 pump had not to be used during an experiment, and, 

 consequently, the two motors worked with greater regularity. 

 The Wolff's bottle was also in direct communication with a 

 Teissier's mercury-pump, by means of which, when working 

 at the lower temperatures, the pressure was greatly reduced 

 near the close of an experiment in order to secure the boiling- 

 off of the last drop of benzene. We found that the mercury- 

 pump had to be kept in constant use during the experiments 

 at 20°. With the exception of the above alterations and 

 additions, the apparatus is the same in every respect as that 

 figured in Plates 4, 5, and 6 of Paper W. 



15. On the Purity of the Benzene. 



The benzene, which was a sample of that used by Professor 

 Ramsay and Miss Marshall for their comparative experiments, 

 was supplied by Messrs. Kahlbaum of Berlin and guaranteed 

 free from thiophene. It was redistilled twice from phosphoric 



* The grease on the core of taps traversed by benzene was replaced by 

 phosphoric acid. 



