ae eS Oe—EE——E——EEEEE a | 
“gaat eos 
<i 
Depolari- 
zation of 
heat. 
(719.) 
M. Reg- 
nault. 
(720.) 
Skill of 
French ex- 
perimen- 
ters. 
(721.) 
Co-ef- 
“ficients of 
expansion 
of gases. 
Cuar. VL, § 9.] 
loni had announced did not take place) ;! next, by 
transmission through a bundle of very thin mica 
plates, inclined to the transmitted ray ; and after- 
wards by reflection from the multiplied surfaces of a 
pile of thin mica plates placed at the polarizing angle.” 
I next succeeded in showing that polarized heat is 
subject to the same modifications which doubly re- 
fracting crystallized bodies impress upon light, by 
suffering a beam of heat (even when quite obscure), 
after being polarized by transmission, to pass through 
a depolarizing plate of mica, the heat traversing a 
second mica bundle before it was received on the 
pile. As the plate of mica used for depolarization 
was made to rotate in its own plane, the amount of 
heat shown by the galvanometer was found to fluc- 
HEAT.—M. REGNAULT. 
957 
tuate just as the amount of light received by the eye 
under similar cireumstances would have done. This 
experiment which, with the others just mentioned, 
was soon repeated and confirmed by other observers, 
still remains the only one proving the double refrac- 
tion of heat unaccompanied by light; and though 
somewhat indirect, it will hardly be regarded by 
competent judges as otherwise than conclusive. Ice- 
land spar and other doubly-refracting substances, 
absorb invisible heat too rapidly to be used for effect- 
ing directly the separation of the rays, which requires 
a very considerable thickness of the crystal. I also 
succeeded in repeating Fresnel’s experiment of pro- Circular. 
ducing circular polarization by two internal reflec- polariza- 
tions. The substance used was of course rock-salt.* 
§ 9. M. RegNauLt.—Numerical Laws of Expansion by Heat ; Rudberg,—Vaporization ; 
Dulong.—Latent Heat ; Hygrometry. 
Tue limits of this Essay will not permit me to do 
more than allude in very general terms to the merito- 
rious services of M. Henrt-Victor Reenavtt in the 
science of heat. In the seventh section of this chapter 
I haye mentioned his name in connection with that 
of Dulong, whose researches he has prosecuted, and 
whose position in the Collége de France he now fills. 
The attention of M. Regnanlt has been devoted 
chiefly to heat in its combinations with matter—to 
dilatation and vaporization. I have already said, in 
speaking of Dulong, that, in point of numerical pre- 
cision in the results of experimental physics, the 
French are unrivalled. The talent which they have 
shown in the construction of apparatus, skill in its use, 
and patience in deducing results with due attention to 
every numerical correction, have not been equalled 
either in England or Germany, much less elsewhere. 
We must, however, note that doubts were first 
tion. 
thrown upon the accuracy of Gay-Lussac’s coefficient Coefficient 
of the expansion of the gases (0°375 of the vo- 
lume at 32° for the expansion between 32° and 212° 
Fahrenheit) by Rudberg a Swedish philosopher, who 
determined a new coefficient (0°3645). M. Reg- 
nault finds for air of the ordinary density a co- 
efficient nearly the same as that of Rudberg, but 
differing slightly for the same fluid under differing 
pressures, and also for the various gases, The ex- 
pansibility of all of these fluids appears to tend to 
the same limiting value when they are sufficiently 
attenuated, As a preliminary to these experiments, 
the expansion of mercury was ascertained by its hy- 
drostatic equilibrium at different temperatures, as 
had already been done by Dulong, and with almost 
coincident results, The dilatation of mercury was 
used to ascertain that of the glass vessels employed. 
of expan- 
sion of 
gases and 
mercury ; 
Rudberg 
—Dulong. 
The irregularity of the dilatation of glass is one of (722. 
1 Annales de Chimie, tom. lv. (1833). 
2 I was led to polarize heat by transmission through mica films from having observed the extraordinary permeability of those 
films to radiant heat, and from the facility of adapting them to tubes applied to the pile. The idea of using bundles of mica for 
reflecting heat did not occur to me until some time after. But I cannot here omit mentioning a circumstance of which I only 
became aware some years after the publication of my researches. In arranging my correspondence, I found some letters from 
Sir David Brewster, with whom I had communicated as to the best means of polarizing heat, during my earliest and unsuccessful 
attempts with common thermometers. In one of these letters he recommends, among other methods, the reflection of radiant 
heat from mica bundles. This suggestion was not put in practice; for, owing to change of residence and other circumstances, 
my attention was diverted to other subjects, and only recalled, after a Japse of some years (as stated in the text), to the polari- 
zation of heat, by the invention of the Thermomultiplier. Nor was Sir D. Brewster’s suggestion recollected by me until I ac- 
cidentally met with it (after another long interval), in the manner which [ have just stated. Lam glad to have an opportunity 
of acknowledging the friendly assistance and encouragement in all matters of science which at an early age I received from him 
when I was an obscure, though ardent student, and when he was my only scientific adviser. 
3 T have not thought it proper to go into farther details concerning my own experiments on radiant heat. Those who desire 
more information will find it in Professor Powell’s Second Report on Radiant Heat, in the Brit. Assoc. Reports for 1840, But I 
my here state, that M. Melloni’s first experiments on polarization were made with mica piles, furnished to him by myself in 1835. 
M. Regnault’s experiments were published in 1841. Professor Magnus of Berlin was at the same time engaged on similar 
experiments, and with nearly coincident results. The following table contains the summary of all these experiments :— 
Dalton’s coefficient ......ceccerseerseaee Radopasteves eivey dd anceves ca¥scanvevecwesandasty sks who buliconceor tapes +++ O°391 
Gay-Lussac’s coefficient ., cr «» 0°375 
Rudberg’s coefficient ........re00++ 5 +. 0°3645 
M. Regnault’s coefficient (from the expansion observed under a constant pressure)... .. 0°367 
M. Regnault’s coefficient (from the elasticity observed under a constant volume)............ 0°3665 
. M. Magnus’ coefficient (from the elasticity observed under a constant volume)............... 0°3665 
Dalton’s experiments were made. between 55° and 212°, and after allowing for the expansion of glass, he obtains for the relative 
volumes of air at those temperatures 1000 and 1325, giving z}, of the volume at 55° for 1° Fahr., or z¢y of the volume at 32°, 
which agrees with the co-efficient given above.. See Manchester Memoirs, vol. v., p. 598-9. 
