ON THE MEASURES AND THE NATURE OF HEAT. 647 



-comparison of all others, is too laborious and complicated for common use. 

 If we mix together equal quantities of the same liquid at two different tem- 

 peratures, they will obviously acquire an intermediate temperature, which is 

 the natural mean between the separate temperatures, provided that no heat 

 be lost or gained during the process ; and provided that no irregularity be 

 produced from the approach of the liquid to a state of congelation, the exist- 

 ence of which might be detected by a comparison of experiments on various 

 liquids at the same temperatures. 13y repeating the operation, we may subdi- 

 vide the interval* as often as we please, or we may mix the liquids in any 

 other proportion, so as to obtain at once any otheripoint of the scale, which 

 may afterwards be identified by a thermometer of any description. 



There is also another method of comparing thcdivisions of a thermometer 

 with those of the natural scale, but it is not wholly free from objections; the 

 instrument being placed in a cone of the sun's rays, made to converge by 

 means of a lens or mirror, the quantity of lieat falling on it must be nearly 

 in the inverse proportion of the square of its distance from the focus j and 

 the elevation of a common thermometer appears to be nearly proportional to 

 the lieat which is throwft_ott It ia .this manner. 



The expansion of solids probably approaches the nearest to the steps of the 

 natural scale, although even in this there seems to be some inequality; but 

 that of mercury is scarcely Jess regular, and a portion of mercury inclosed 

 in a bulb of glass, having a fine' tube connected with it, forms a thermometer 

 the most convenient, and most probably the mostaccuratc,of any, for common 

 use; the degrees corresponding very nearly with those of the natural scale, 

 although, according to the most accurate experiments, they appear to indi- 

 cate, towards the middle of the common scale of Fahrenheit, a temperature 

 2 or 3 degrees too low. There is an inequality of the same kind, but still 

 greater, in the degrees of the spirit thermometer; and this instrument has 

 also the disadvantage of being liable to burst in a heat below that of boiling 

 water; although it is well calculated for the measurement of very low tem- 

 peratures, since pure alcohol has never yet been frozen, while mercury has 

 been reduced to a solid by the cold of Siberia and of Hudson's Bay: but both 

 mercury and linseed oil support a heat of between .5 and 600° without ebul- 

 lition. For higher temperatures than this, a thermometer has been made of 



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