THERMOMETER AND PYROMETER. 



53 



from the cause just alluded to, and the 

 impossibility of altogether obviating the 

 effects of currents of air through it, 

 during experiments, are not susceptible 

 of such accuracy as the more simple 

 method of mixture ; although this also 

 is liable to lead to erroneous conclusions, 

 from the difficulty of obtaining the true re- 

 sult of the effect of the different mixtures. 



2. There is an obvious source of 

 error in the indications of all instru- 

 ments employed to measure tempera- 

 ture, in as much as the apparent changes 

 in the volume of the thermometric sub- 

 stance are not the real augmentation or 

 diminution of bulk it undergoes. The 

 glass of the thermometer, and the frame 

 of the pyrometer, are also expanded 

 and contracted by changes of tempera- 

 ture ; so that our instruments only show 

 the excess of the expansions of the ther- 

 mometric fluid, or pyrometric bar over 

 those of the glass and frame ; by which 

 the true indications are diminished. 



From the extreme nicety of some of 

 the investigations in which the mercu- 

 rial thermometer is employed, a compa- 

 rative ratio of the expansions of mer- 

 cury and glass has been most diligently 

 sought after by De Luc, Ramsden, Roy, 

 and others. From their investigations 

 it has been ascertained that all solids 

 and liquids vary in their rate of expan- 

 sion ; but that the expansibility of glass 

 depends so much on the manufacture 

 of that article, and varies so much in 

 the different kinds, that no general 

 equation as a correction for this source 

 of eiTor can be of practical utility. Even 

 the form of the glass rod is material. 

 Roy gives the expansion of a glass 

 tube = 0. 0046569th ; of a solid glass 

 rod = 0.0096944th, in passing from the 

 freezing to the boiling point of water.* 



An Important series of experiments 

 by Lavoisier and Laplace have been 

 published by Biot,-i- from which it ap- 

 pears that of twenty-three solids tried, 

 glass was the least expansible of them all. 



The length of different glass rods, 

 which at 32 Fahrenheit= 1.00000000, at 

 2 12 Fahrenheit is augmented as follows: 



Glass of St. Gobain 1 .00089089 T / 25 



Glass tube, without tead 1 .00087 572= TT ^ 5 



Ditto 1.00089760r= TT ' T 5 



Ditto 1.00091751= Ts y 5 ' 



French glass, with lead ...1 . 00087 199r= TT - y 

 English flint glass 1 .00081166 ~ 



This will show the impossibility of 

 any general correction being applied ; 

 and the adoption of any formulae for 

 this purpose would be an affectation of 

 accuracy, of which, unfortunately, the 

 subject is incapable. If, however, such 

 formula is considered desirable, it may 

 easily be constructed from the experi- 

 ments of De Luc on glass tubes $ at 

 different temperatures, which, reduced 

 to the scale of Fahrenheit, are, 



Temp. 



32 



50 



70 

 100 

 120 



Bulk. 

 100000 

 100006 

 100014 

 100023 

 100032 



Temp. 



150 



167 



190 



212 



Bulk. 

 100044 

 100056 

 100069 

 100083 



* Phil. Trans., vol. Ixxv. 



f Traite de Physique, t. ii. 153. 



3. Another error of some magnitude 

 is produced by the inequalities of the 

 expansions of the same substances by 

 equal increments of temperature. 



If we could consider expansion simply 

 as the effect of the application of heat, 

 equal increments of temperature should 

 produce equal rates of increase of vo- 

 lume ; but the expansion is the re- 

 sultant, in solids and in liquids, of two 

 opposite forces of the repulsive energy 

 of caloric opposed by the cohesion of 

 the particles of matter; and, accord- 

 ingly, it not only differs in the different 

 kinds of solid and liquid matter, but 

 in the same body at different tempera- 

 tures. As might be expected from this 

 view, it must be in an increasing ratio 

 with the temperature ; because the force 

 of cohesion must diminish with the dis- 

 tance of the particles of matter. 



In aeriform bodies, the force of cohe- 

 sion does not exist ; and we might in- 

 fer that equal increments of heat 

 would produce equal expansions, in all 

 gases, at all temperatures. In gases, 

 the ratio might even be expected to de- 

 crease, in a minute but inappreciable 

 degree, with the temperature ; because 

 the increased distance of the particles 

 will tend to diminish the repulsive 

 energy. 



Experiment in these particulars ac- 

 cords with theory. The ratio of expan- 

 sion in solids and liquids is found to be 

 an increasing one, as the temperature 

 is augmented, and is very different in 

 each substance ; while in the gases, it is 

 not only equable in the same gas, but 

 equal in all. 



The manner in which an increasing 



t Recherches, t, i. 



