80 THERMOMETRY ACCURACY. 



sophers have subsequently directed their attention to the 

 subject. Dalton was, however, the first who succeeded in 

 measuring the change of temperature with some degree of 

 accuracy. By the employment of an exceedingly ingenious 

 contrivance, that illustrious philosopher ascertained that 

 about 50 of heat are evolved when air is compressed to 

 half of its original bulk, and that on the other hand 50 are 

 absorbed by a corresponding rarefaction." 



This paragraph, besides showing that Joule had studied 

 the determinations as to the thermal properties of gas then 

 existing, is also interesting as containing the first reference 

 which he has had occasion to make to the works of his 

 great master. He then proceeds : — 



" There is every reason for believing that Dalton's 

 results are very near the truth, especially as they have been 

 exactly confirmed by the experiments of Dr. Ure, with the 

 thermometer of Breguet." " But," he continues, " our 

 knowledge of the specific heat of elastic fluids is of such an 

 uncertain character that we should not be justified in 

 attempting to deduce from them the absolute quantity of 

 heat evolved or. absorbed. I have succeeded in removing 

 this difficulty by immersing my condensing pump and 

 receiver into a large quantity of water, so as to transfer 

 the calorific effects to a body which is universally received 

 as the standard of capacity." Then, after describing the 

 pump, he proceeds : " Anticipating that the changes of 

 temperature of the large quantity of water which was 

 necessary to surround the pump and receiving apparatus 

 would be very minute, I was at great pains in providing a 

 thermometer of extreme sensibility and very great accuracy. 

 A glass tube of narrow bore having been selected, a column 

 of mercury, 1 inch long, was introduced, and gradually 



