18 



HEAT. 



Dr. Murray proposed a scale which he 

 believed would combine all the advan- 

 tages of other scales, without their dis- 

 advantages ; his plan is to make the 

 extreme points of his scale at the freez- 

 ing and boiling of mercury, and to di- 

 vide the space between into 1000 de- 

 grees. 



As mercury expands more uniformly 

 than any other fluid, and as there is a 

 wider range between its freezing and 

 boiling points, mercurial thermometers 

 are most generally useful ; but, when low 

 degrees of temperature are to be mea- 

 sured, a thermometer containing co- 

 loured alcohol is the best adapted to 

 the purpose, since it has not been found 

 possible to make this fluid solid by any 

 reduction of temperature yet effected. 



Dr. H alley, Dr. Brook Taylor, Dr. 

 Black, and other philosophers, have 

 made numerous experiments to ascertain 

 whether the expansion and contraction of 

 mercuiy is the same at every tempera- 

 ture, for equal portions of heat applied 

 or withdrawn. The experiment most ge- 

 nerally made by them was to take a ther- 

 mometer with a perfectly cylindrical 

 tube, and having plunged it in hot wa- 

 ter, to mark the stationary point of the 

 fluid ; they then observed where the fluid 

 stood when plunged into an equal weight 

 of cold water ; and, lastly, they mixed 

 the two portions of water together, and 

 tried the temperature -of the mixture : 

 if the temperature indicated was the 

 mean between the temperatures of the 

 two fluids, they considered the indica- 

 tions of the thermometer, as to changes 

 of temperature, to be correct. Many 

 sources of error were to be guarded 

 against in this experiment, and different 

 conclusions were arrived at ; De Luc's 

 opinion was, that the thermometric fluids 

 do not expand equally, with equal quan- 

 tities of heat applied. The least devia- 

 tion from regularity was observed in 

 mercury. Equal weights of water at 

 200'7 and 45 being mixed together, the 

 temperature was 2-5 lower than the 

 arithmetical mean, and experiments tried 

 with other temperatures gave similar 

 results. 



De Luc thought that experiments of 

 the kind just stated could not be relied 

 upon, as they rested on an assumption 

 that the capacity of water for heat is 

 the same throughout the whole range of 

 temperature operated upon, while he 

 believed that the capacity increased 

 along with the temperature. On mix- 

 ing two quantities of water at different 



temperatures, a diminution of capacity 

 would therefore result, and heat would 

 consequently be given out, which would 

 occasion the temperature of the mix- 

 ture to appear higher than it ought 

 to do. 



Dr. Crawford, in order to obviate this 

 objection, exposed a thermometer 

 equally to air cooled by snow to 32, 

 and heated by steam to 212: the re- 

 sulting temperature at which the in- 

 strument remained stationary fifteen 

 minutes, was 121, only one degree 

 lower than the mean ; and he thought 

 that this deviation should be reduced, by 

 admitting a correction for the effect of 

 temperature on the quantity of fluid in 

 the stem. 



Experiments of a still greater ac- 

 curacy have been made by Dr. Ure, 

 and by MM. Dulong and Petit, which 

 prove that the slight degree of inequa- 

 lity in the expansion of mercury in 

 thermometer-tubes is compensated by 

 the expansion of the glass, and also by 

 the lessening mass of mercury remain- 

 ing in the bulb as the temperature 

 rises ; so that the mercurial thermome- 

 ter may be considered as an accurate 

 indicator of changes of temperature. 



Various modifications of the air ther- 

 mometer have been introduced, of which 

 Mr. Leslie's is considered the most use- 

 ful*. It is called the differential ther- 

 mometer, and consists of a long glass 

 tube, twice bent at right angles, having a 

 bulb at each extremity ; the tube contains 

 a quantity of sulphuric acid tinged with 

 carmine. The original adjustment of this 

 fluid is rather difficult, and requires care 

 and dexterity. The instrument is fur- 

 nished with a scale of 1 00 equal parts, 

 and is fixed upon a wooden support. 

 Both the bulbs of the instrument being 

 exposed to the same temperature, it is 

 not in the least affected ; but as soon as 

 one of the bulbs is exposed to a higher 

 temperature than the other, the difference 

 between them is delicately shewn by the 

 falling of the coloured fluid below the 

 bulb which is most heated. This in- 

 strument not being affected by the vari- 

 ations of atmospherical pressure, nor by 

 fluctuations of temperature in the at- 

 mosphere, it is admirably fitted for 

 experiments on radiant heat ; its form 

 is shewn aty?g\ 7. 



Another modification of this instru- 

 ment was introduced by Dr. Howard, 



* Experimental Inquiry into the Nature and Pro- 

 pagation of Heat, 



