DILATATION OF THE GASES. 213 



fixed point, he compared the heights found by the logarithms 

 with thofe which he had meafured, by attributing to heat the 

 excefles and deficiences of the firfl for the fecond ; and he 

 concludes " that near the fixed temperature, the correction Correction for 

 for a degree of the thermometer is to the height of the place temperature. 

 : : 1 : 21 3" (Becker, fur les modi/, de Vat. IV. part. ch. III.) 



Colonel Roy difcovered a much greater dilatation in air. Colonel Roy's 

 According to him near the 15° of the thermometer divided ^°^" cc 

 into 80 parts, the air dilates T fj of its volume for each de- 

 gree. He likewife found that humid air dilates much more 

 than dry air; but Sauflure obferves, that Colonel Roy having 

 introduced into his manometer either liquid water, or the 

 vapor of water, confounded two things which ought to be but he produced 



feparated, namely, the converfion of water into an elaftic morc va P? r , the 

 n .. ,,,.,- r • •»•,/-. ,™ n greater his heat, 



fluid, and the dilatation of air united with luch vapor. (Phil. 



Tranfaft. 1777, p. 704.) 



Sauflure fixes the dilatation of air near the 6th degree, at De SaufTure's 

 ¥ -|t °f ^ s volume for each degree. His experiments were "fait, 

 made in a great receiver, "in which were enclofed a thermo- 

 meter and barometer, to indicate the Variations of the tem- 

 perature of the air, and the correfpondent elafticity it would 

 acquire. To difcover the influence of water on the dilata- 

 tion of air he enclofed in his receiver air more or lefs dry, and 

 avoided the production of new vapors. He was fo far from He avoided the 



finding this air more dilatable than vef y dry air, that he on the P roduftion of 



° . . new vapor j and 



contrary thought very dry air was a little more dilatable than in this cafe 



very moift; but always holding its water in perfect folution. founddr y air ra- 

 ,-L . „ ' . °. r ther more dilat- 



[EJfai fur I Hygrometrie, p. 108.) able. 



Till this period philofophers had confined themfelves to the The gafes were 



dilatation of atmofpheric air, and the firit who examined that 110 **?* exa " 



1 mined. 



of the other gafes was the celebrated Prieftley. The following Prieftley's expe- 

 ls his procefs : riments. 



After having filled a phial of gas over mercury, he adapts The gas was re- 

 to it a bended tube, one of its branches being very inclined, ceived over mer- 

 ,and he leaves a little of the mercury in the neck of the phial, e x P anfion no- 

 that the expanfion of the gas may make it pafs into the tube, ticed in a re- 

 This done, he places his apparatus in a box of wood, joins a curve tu e# 

 thermometer with it, and takes it into apartments of different 

 temperatures. The dilated air caufes the mercury to pafs 

 through a greater or lefs fpace in the tube, and by this fpace 



meafured 



