108 Mr. J. H. Alexander on the Tension of Vapour qfWat€7\ 



Pressures in 

 atmospheres. 



0047368 



0006 



0000684 



Temperatures (Centigrade). 



Observed by 

 Regnault. 



32-38 



0-00 



-25-00 



Calculated by formulae of 

 Dulong. Franklin Instit. Alexander. 



36-16 



1045 



- 7-25 



3352 



4-28 



-17-31 



29-80 

 - 108 

 -23-89 



The last two columns are added here for illustration ; and 

 show, among other things, that the formula of the Franklin 

 Institute is, like that of the French Academy, inapplicable to 

 low temperatures and pressures. 



Later than these, M. Biot, in 1839, propo.sed another for- 

 mula, and in IB^l published a table calculated by it, in which 

 the pressures are given in metres and for every degree Centi- 

 grade from —20° to 220° C, corresponding to the limits of 

 — 4° and 428° F. The patient labour requisite for this task 

 has not been overrated by its distinguished performer ; as can 

 be readily appreciated, when it is known that part of the cal- 

 culations actually were, and it was apprehended that even the 

 whole might require to be, executed with logarithms of eleven 

 decimals, and that the constants reach even the twelfth deci- 

 mal place. These constants were derived, for the higher 

 temperatures, from the already quoted experiments of Dulong 

 and Arago and of Taylor ; and for the lower, from unpub- 

 lished experiments of M. Gay-Lussac. The temperatures 

 are throughout given in terms of an azV-thermometer instead 

 of a mercurial one, a modification which undoubtedly im- 

 presses a more systematic accuracy upon the method ; but yet, 

 in spite of the aid afforded by tabular corrections for reduc- 

 tion, appears to diminish materially the chances of practical 

 resort to the table itself. These temperatures M. Biot, in the 

 form first proposed by Prony, (the same which Dr. Young, 

 with more emphasis than reflection, has called "ridiculously 

 complicated,") employs as the exponents of a series; the pe- 

 culiarity of the method, however, is in that the direct nume- 

 rical result of the equation gives, instead of the pressure itself, 

 the tabular logarithm of the pressure. It is therefore essen- 

 tially a logarithmic formula. 



1 present the following comparison between it and the pre- 

 sent formula, applied to the same instances of experiment, 

 which have been already signalised by M. Dulong himself, 

 and already quoted here. To save both the tedium and ha- 

 zard of a reduction to English measures, 1 leave the quantities 

 under their original denominations ; and, in so far varying 

 from the preceding instances, I give the deviation of the for- 



