316 Cambridge Philosophical Society. 



ceconomy being satisfied, the machine is so arranged that at the close 

 of a complete operation the substance (air in one case and water in 

 the other) employed is restored to precisely the same physical con- 

 dition as at the commencement. He thus shows on what elements, 

 capable of experimental determination, either with reference to air, or 

 with reference to a liquid and its vapour, the absolute amount of 

 mechanical effect due to the transmission of a unit of heat from a 

 hot body to a cold body, through any given interval of the thermo- 

 metry scale, may be ascertained. In M. Clapeyron's paper various 

 experimental data, confessedly very imperfect, are brought forward, 

 and the amounts of mechanical effect due to a unit of heat descend- 

 ing a degree of the air-thermometer, in various parts of the scale, 

 are calculated from them, according to Carnot's expressions. The 

 results so obtained indicate very decidedly, that what we may with 

 much propriety call the value of a degree (estimated by the mecha- 

 nical effect to be obtained from the descent of a unit of heat through 

 it) of the air- thermometer depends on the part of the scale in which 

 it is taken, being less for high than for low temperatures*. 



The characteristic property of the scale which I now propose is, 

 that all degrees have the same value ; that is, that a unit of heat 

 descending from a body A at the temperature T° of this scale, to a 

 body B at the temperature (T— 1)°, would give out the same me- 

 chanical effect, whatever be the number T. This may justly be 

 termed an absolute scale, since its characteristic is quite independent 

 of the physical properties of any specific substance. 



To compare this scale with that of the air-thermometer, the values 

 (according to the principle of estimation stated above) of degrees of 

 the air-thermometer must be known. Now an expression, obtained 

 by Carnot from the consideration of his ideal steam-engine, enables 

 us to calculate these values, when the latent heat of a given volume 

 and the pressure of saturated vapour at any temperature are experi- 

 mentally determined. The determination of these elements is the 

 principal object of Regnault's great work, already referred to, but at 

 present his researches are not complete. In the first part, which alone 

 has been as yet published, the latent heats of a given weight, and the 

 pressures of saturated vapour at all temperatures between 0° and 230° 

 (Cent, of the air- thermometer), have been ascertained ; but it would 

 be necessary in addition to know the densities of saturated vapour 

 at different temperatures, to enable us to determine the latent heat 

 of a given volume at any temperature. M. Regnault announces his 

 intention of instituting researches for this object ; but till the results 

 are made known, we have no way of completing the data necessary 



* This is what we might anticipate, when we reflect that infinite cold 

 must correspond to a finite number of degrees of the air-thern>ometer below 

 zero ; since, if we push the strict principle of graduation, stated above, suf- 

 ficiently far, we should arrive at a point corresponding to the volume of 



air being reduced to nothing, which would be marked as — 273° ( — ■ .,. , 



■«joo 



if -366 be the coefficient of expansion) of the scale ; and therefore —273° 



of the air-thermometer is a point which cannot be reached at any finite 



temperature, however low. 



