40 Miss Marshall and Dr. Ramsay on the Heats of 



of the published values with which we were acquainted, 

 Mr. E. H. Griffiths, F.R.S., kindly consented to make a 

 determination of the Heat of Evaporation of Benzene by 

 the same method that he used with such conspicuous success 

 in the case of water. 



His experiments (in which I also took part) measured the 

 heat of evaporation over the range 30° to 50° C. ; and gave 

 by extrapolation the value at 80 o, 2, the boiling-point. This 

 number is therefore the one finally adopted by us in the 

 calculation of our results ; and we take this opportunity of 

 recording our sincere thanks to Mr. Griffiths for furnishing 

 us with such a trustworthy standard. 



It should be observed that this number finally obtained for 

 the heat of evaporation of benzene at its boiling-point does 

 not in any way affect our work regarded as a method of 

 comparison ; but it does enter as the most important feature 

 into the calculations by which we pass from our ratios to the 

 absolute values of the heats of evaporation, and bring our 

 results into line with those of other observers.] 



4. Description of the Apparatus. 



In the first experiments vessels were used closely resem- 

 bling the ordinary incandescence-lamps, with carbon filaments; 

 but a good deal of trouble was caused by the extreme diffi- 

 culty of making satisfactory connexions. After the prelimi- 

 nary experiments, these lamps were therefore rejected in 

 favour of others which had a spiral of fine platinum wire in 

 place of the carbon filament : the ends of the spiral were 

 attached to stout platinum terminals sealed into the glass ; 

 the terminals were gilt and amalgamated, and thus a safe and 

 good contact was secured. The upper part of each lamp was 

 drawn out into a rather narrow open tube, through which the 

 liquid could be introduced, and which could be securely closed 

 to prevent loss during weighing. The lamp was set up in an 

 ordinary vapour-jacketing arrangement provided with side- 

 bulb and condenser. The jacket was closed at the bottom by 

 an indiarubber cork, through which passed two U-tubes con- 

 taining mercury ; the terminals of the lamp rested on the 

 inner ends, into the outer ends dipped the wires carrying the 

 current. The cork was protected by a layer of mercury so 

 that it could not come into contact with the liquid. Each 

 lamp was jacketed with the vapour of its own liquid, so that 

 the temperature of its contents would be raised to the boiling- 

 point without the possibility of ebullition taking place until 

 the current was started ; the liquid then lost by boiling from 



