the Vapour of Sulphuric Acid, 



503 



this bath is heated by gas-jets G; Ta thermometer suspended 

 in the bath to determine its temperature when the vapour is 

 being formed in the upper portion of the tube AB, coincident 

 with the column of mercury in the barometer-tube D E. The 

 tube being filled just beyond the bending C with the liquid to 

 be experimented upon, the air is expelled from the liquid by 

 continuous ebullition and exhaustion. It will be observed that 

 whilst the upper portion of the acid in the tube is being heated, 

 the lower portion, owing to the slow conducting-power of the 

 liquid, remains nearly at a constant temperature. The pressure 

 of the vapour when formed is found by adding the column of 

 acid in the tube ABC, converted into inches of mercury, to the 

 column of mercury in the barometer-tube, and then subtracting 

 this sum from the barometric column, measuring the pressure of 

 the atmosphere at the time of observation. After experiment- 

 ing with strong sulphuric acid having a specific gravity of 1/846, 

 this acid was diluted with different, equivalents of water, and 

 then these diluted acids were successively subject to experi- 

 ment. The following Table gives the results of one of these 

 experiments : — 



Experiment. — Elasticity of the Vapour of strong Sulphuric 

 Acid diluted with one equivalent of Water, the formula of 

 composition being SO 3 HO + HO. 



Temperature 

 in degrees F. 



T. 



Corresponding 



pressure of 



vapour in inches 



of mercury. 



P- 



Value of p by 

 formula 



logp-6-307- 3062 . 

 5 T+230 



Value of p from 



the formula 



p=p'. 



290° 

 320 

 350 

 370 

 390 

 400 



215 



5-00 

 1000 

 1600 

 23 04 

 28-00 



2-62 

 5-49 

 10-65 

 15-98 

 23-34 

 28-00 



1-89 



4-48 



9-61 



15-28 



23-45 



28-75 



Here the results in the third column show that the proposed 

 formula very nearly represents the results of experiment : similar 

 formulae were found to represent the results of all the other expe- 

 riments. In the fourth column, p 1 is put for the pressure of the 

 vapour of water at a temperature of 190° below the correspond- 

 ing temperature given in the first column : thus, for example, 

 15*28 is the pressure of the vapour of water at 180°, that is, 

 190° below 370°, the corresponding temperature in the first 

 column : the near coincidence of the results in the fourth and 

 second columns shows that this law (known as Dalton's law of 



