Mr Tregaskis o?t the Ka^pansion of Vapour. 69 



of this comparison is, that one-fifth added to any given por- 

 tion of heat already communicated to water, as indicated by the 

 thermometer ^rom the freezing point., will double the elastic force 

 of its vapour. The annexed table is calculated on this principle, 

 and the calculation agrees well with experiment from 3Q° be- 

 low the boiling point up to 343° of Fahrenheit, the highest ex- 

 periment hitherto pubhshed. This law is easily reducible to 

 a geometrical ratio for each factor. The ratio of force being 2, 

 we have only to reduce 14 to the decimal 1.2 for the ratio o 

 temperature. Having found the ratio, it is easy to calculate 

 the force of vapour at any given temperature, and vice versa, 

 (provided the same ratio answers, while vapour retains the 

 same physical condition, (which I think will not be questioned,) 

 viz. from the freezing point upward till vapour is changed into 

 permanent gas.) For, by counting the number of terms in 

 each series produced by the continual multiplication of both 

 factors by their respective ratios, the corresponding tempera- 

 ture and force is seen at once. For example, — 



1st. 2d. 3cl. 4th. 



Temp. 180 x l.^:=z91Q X 1.2 = 259.2 X 1.2 = 311.04. 



Force, 30 X 2 = 60 x 2=120 x2 =240. 

 And by adding 32 to either of the terms in the series of tem- 

 perature, we have the degree of Fahrenheit. For instance, at 

 the fourth term we have for temperature 311*^.04, force 240. 

 311°.04 -I- 32° = 343°.04, so that the force of vapour by calcu- 

 lation at 343°.04 of Fah. supports 240 inches of mercury, and 

 at 343°.6 of Fah. it supports 240 inches by Mr Southern's ex- 

 periment. 



From this it appears that the calculation answers Mr S.'s 

 experiment to the fraction of one degree on 343°. The fourth 

 line in the annexed table is nearly a mean between the expe- 

 riments of Ure and Southern. The third agrees with that of 

 Dr Ure to a small fraction. The next term under the boiling 

 point agrees to the fraction of an inch with Mr Dalton. And 

 M. Betancourt's statement, that vapour at 182° has half the 

 tension at 212°, agrees with the table exactly. 



It will be observed that the experiments I have selected are 

 in that part of the thermometric range which is most satisfac- 

 tory, viz. from 182° upwards. In experiments near the freez- 



