67 



searches* ; and we are thus at present in possession of as complete a 

 practical solution of the problem as can be desired, even for the most 

 accurate investigations. The theory of thermometry is however as 

 yet far from being in so satisfactory a state. The principle to be 

 followed in constructing a thermometric scale might at first sight 

 seem to be obvious, as it might appear that a perfect thermometer 

 would indicate equal additions of heat, as corresponding to equal 

 elevations of temperature, estimated by the numbered divisions of 

 its scale. It is however now recognized (from the variations in the 

 specific heats of bodies) as an experimentally demonstrated fact that 

 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 f. This remarkable 

 circumstance enhances very much the practical value of the air- 

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



* A very important section of Regnault's work is devoted to this object. 



f 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 are experimentally determined for each thermometer. 



