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



2. Description of the Method. 



In order to render our account of these experiments in- 

 telligible, we find it necessary to give the following somewhat 

 lengthy extract from Paper W, pp. 270-273 : — 



" The method adopted was of such a nature that the results 

 would not be appreciably affected by 



(1) errors in thermometry ; 



(2) changes in the specific heat of water ; 



(3) the capacity for heat of the calorimeter ; 



(4) loss or gain of heat by radiation, &c. ; 



and if these points are borne in mind, they may serve to 

 explain some of the contrivances which might otherwise 

 appear uncalled for. 



" If the vessel in which the evaporation is taking place is 

 kept at a constant temperature, we are independent of the 

 capacity for heat of it and its contents ; we also dispense 

 with the measurements of changes of temperature. Thus, if 

 matters be so arranged that the loss and gain of heat through- 

 out an experiment are balanced, many fruitful causes of 

 error are avoided. Of course, the actual temperature of the 

 calorimeter during evaporation must be determined, but a 

 small error here is of little consequence. The change in the 

 value of L is small as compared with the changes in 6. In 

 fact, an accuracy of an order of y 1 ^ of a degree would be 

 sufficient when determining the actual elevation. 



u The heat was supplied to the calorimeter by means of a 

 wire whose ends were kept at a constant potential difference. 

 The thermal balance could be maintained in one of two ways: — 



(1) If the heat-supply was too great, the electric current 



could be temporarily stopped, or, the rate of evapo- 

 ration of the water increased. (The latter was the 

 method generally adopted.) 



(2) If the cooling was too rapid, the only mode of main- 

 taining the balance was (in the apparatus about to be 

 described) to reduce the rate of evaporation. 



" The liquid to be evaporated was contained in a small 

 silver flask, connected with which was a spiral coil of silver 

 tubing 18 feet in length. Both flask and spiral were within 

 the calorimeter, and the water- vapour, after passing through 

 the spiral, emerged from the apparatus at the temperature 

 of the calorimeter. Surrounding the flask, and between it 

 and the spiral, a coil of platinum-silver wire was arranged, 

 and flask, spiral, and coil were entirely immersed in a certain 

 singularly limpid oil consisting of hydrocarbons only. 



