100 Dr. Masons Description 



tion, until it has attained its utmost limits, the tension of 

 the vapour given off depending upon the temperature of 

 the evaporating surface, until the further refrigeration is 

 finally checked by meeting with vapour of a similar elas- 

 ticity existing in the atmosphere. 



I think this opinion more consistent, and will better ac- 

 cord with experimental results than the law advanced by 

 Professor Daniell, namely, " The tension of vapour given off 

 in the process of evaporation is determined, not by the tem- 

 perature of the evaporating surface, but by the elasticity of 

 the aqueous atmosphere already existing." Professor 

 Daniell afterwards observes that this takes place only 

 within a limited circle, " the term limited circle" implies 

 an exception to a general law, and is therefore objection- 

 able. Now, if the tension of the vapour given off in the 

 process of evaporation depended upon the elasticity of that 

 already existing in the atmosphere, supposing vapour of 

 only one tension to exist, it is by no means clear how the 

 Dew-point could ever vary, or why the temperature of the 

 moistened bulb of a thermometer does not correspond with 

 the temperature of the Dew-point in the process of evapo- 

 ration ; when the tension of the vapour at the Dew-point 

 sets limits to the process of evaporation at that tempera- 

 ture ; because it is quite clear, that the tension of vapour 

 given off at any higher temperature would overcome that of 

 any other inferior to it. 



From my experiments on the subject, I am led to the 

 belief that the tension of the vapour given off depends upon 

 temperature, and that the process of evaporation would be 

 arrested at each temperature, by meeting with a similar 

 force of vapour already existing in the atmosphere, pro- 

 vided the atmosphere was fully saturated at that tempera- 

 ture. Hence, it follows that the moistened bulb of a ther- 

 mometer cannot be reduced to the temperature of the Dew- 

 point, as it undoubtedly would have been, had vapour of 

 only one tension existed ; but that it will stand at a tem- 

 perature corresponding to the mean tension of every force 

 of vapour existing from that of the air to the Dew-point. 



As this is an important point, I made repeated experi- 

 ments for the purpose of verifying its accuracy, three of 

 which I may particularize. The density of the atmosphere 



