314 Cambridge Philosophical Society. 



thermometry under this condition is impossible, and we are left with- 

 out any principle on which to found an absolute thermometric scale. 



Next in importance to the primary establishment of an absolute 

 scale, independently of the properties of any particular kind of mat- 

 ter, is the fixing upon an arbitrary system of thermometry, according 

 to which results of observations made by different experimenters, in 

 various positions and circumstances, may be exactly compared. This 

 object is very fully attained by means of thermometers constructed and 

 graduated according to the clearly defined methods adopted by the 

 best instrument- makers of the present day, when the rigorous expe- 

 rimental processes which have been indicated, especially by Regnault, 

 for interpreting their indications in a comparable way, are followed. 

 The particular kind of thermometer which is least liable to uncertain 

 variations of any kind is that founded on the expansion of air, and 

 this is therefore generally adopted as the standard for the comparison 

 of thermometers of all constructions. Hence the scale which is at 

 present employed for estimating temperature is that of the air-ther- 

 mometer ; and in accurate researches care is always taken to reduce 

 to this scale the indications of the instrument actually used, whatever 

 may be its specific construction and graduation. 



The principle according to which the scale of the air-thermometer 

 is graduated, is simply that equal absolute expansions of the mass of 

 air or gas in the instrument, under a constant pressure, shall indicate 

 equal differences of the numbers on the scale ; the length of a " de- 

 gree " being determined by allowing a given number for the interval 

 between the freezing- and the boiling-points. Now it is found by 

 Regnault that various thermometers, constructed with air under dif- 

 ferent pressures, or with different gases, give indications which coin- 

 cide so closely, that, unless when certain gases, such as sulphurous 

 acid, which approach the physical condition of vapours at saturation, 

 are made use of, the variations are inappreciable*. This remarkable 

 circumstance enhances very much the practical value of the air- 

 thermometer ; but still a rigorous standard can only be defined by 

 fixing upon a certain gas at a determinate pressure, as the thermo- 

 metric substance. Although we have thus a strict principle for con- 

 structing a definite system for the estimation of temperature, yet as 

 reference is essentially made to a specific body as the standard ther- 

 mometric substance, we cannot consider that we have arrived at an 

 absolute scale, and we can only regard, in strictness, the scale actu- 

 ally adopted as an arbitrary series of numbered points of reference 

 sufficiently close for the requirements of practical thermometry . 



In the present state of physical science, therefore, a question of 

 extreme interest arises : Is there any principle on which an absolute 

 thermometric scale can be founded} It appears to me that Carnot's 



* Regnault, Relation des Experiences, &c, Fourth Memoir, First Part. 

 The differences, it is remarked by Regnault, would be much more sensible 

 if the graduation were effected on the supposition that the coefficients of 

 expansion of the different gases are equal, instead of being founded on the 

 principle laid down in the text, according to which the freezing- and boiling- 

 points arc experimentally determined for each thermometer. 



