208 DILATATION OF THE GASES. 



Thethermome- The thermometer, as it is at prefent constructed, cannot be* 

 dlcatrthe°reir applied to point out the exad proportions of heat, beeaufe we 

 quantities of are not yet acquainted with the relation between its degrees 

 hcat * and the quantities of heat. It is indeed generally thought 



that equal divitions of its fcale reprefent equal tenfions ot ca- 

 loric ; but this opinion is not founded on any well decided 

 facl. 

 Kumerous de- We mud admit therefore, that we are flill far from pofTef- 

 f a ,a h dS ar!d P the°" fin & P ofltive knowledge refpefting the dilatation of the gafes 

 a ;ts ht the and vapors, and the correfpondent march of the thermometer ; 

 ki owitfdge of nevertheless, we have daily occafion in natural philofophy and 



the cxpanfion of , J r r J 



gas and vapor, chemiftry to reduce a given volume of gas from one tempera- 

 ture to another, to meafure the heat difengaged or abforbed 

 in the changes of conftitution of bodies ; or that difengaged or 

 abforbed by the fame body in palling from one temperature to 

 another ; and in the arts to calculate the efFe&s of fire engines, 

 or to eftimate the expanfions of various bodies; in meteoro- 

 logy to determine the quantity of water held in folution by 

 the air, a quantity which varies with its temperature and its 

 denfity, according to a law not yet afcertained. Laftly, in 

 the conftruclion of tables of aftronomical refraction, and in the 

 application of the barometer to the meafurement of heights, it 

 is indifpenfible that we fhould accurately know the tempera- 

 ture of the air, and the law of its dilatations. 

 The author's Though thefe confiderations were of themfelves fufficient 



thU U work. ,tS t0 to ren( ^ er tne enquiry into an objecl of fuch general utility 

 deferable, yet the difficulty of the refearches it demands would 

 have prevented my attempting it, if I had not been flrongly 

 engaged by Cit. Berthollet, whofe pupil I have the honor to 

 be. I am indebted to him for the neceifary means of exe- 

 cuting this work, in which I have often been aflifted by his 



The firft ferves to introduce water, the fecond admits a thermome- 

 ter to indicate the temperature of the vapor, and to the third is 

 adapted a recurved barometrical tube, to meafure the elafticity of 

 the fame vapor. A vacuum is made in the boiler by means of an 

 air pump communicating through a tube provided with a flop cock. 

 The apparatus of Ziegler differs little from that of Bettancourt; 

 but as Ziegler did not make a vacuum in his boiler, there is a great 

 difference in their experiments and refults. See the Hydraulic 

 Architeaure of Prony, Vol. II.— G. L. 



advice. 



