RECENT PROGRESS IN RELATION TO THE THEORY OF HEAT. 



By a. Cazin. 



Translated for the Smithsonian Institution.* 



The study of heat pi'esents a remarkaLle example of the connection -which 

 exists between the physical properties of matter. Restricted as the limits of 

 this discourse must necessarily he, I propose on the present occasion to consider 

 heat under two points of view only, first in its relations to lii^'ht, and nest in its 

 relations to movement. It may thus be practicable to furnish a rapid sketch of 

 the actual state of this part of physics. 



The fact that the same laws are applicable to the propagation of heat and 

 to that of light, is one which science leaves no longer in doubt. To every expe- 

 riment in optics there corresponds a similar experiment in thermotics. The methods 

 of obsen'ation are to-day carried to such perfection that M. Desains has l)een 

 able quite recently to reproduce with heat the remarkable phenomenon of the 

 luminous interference from striated surfaces. 



A pencil of horizontal luminous rays traverses a plate of glass on which arc 

 traced parallel lines extremely close to one another, (G,000 lines in an extent of 

 one inch.) This pencil is divided by the plate into many distinct pencils, 

 which spread themselves in fan-shape in a horizontal plane, and we see, on the 

 screen upon which these pencils are made to fall alternate intervals of light and 

 obscurity. With the violet light the dark intervals are not so large as with the 

 red. This is the phenomenon of diffraction discovered by Frauenhofer. By 

 applying a thermo-electric battery, the most delicate of thermometers, at different 

 places in the plane where the pencils transmitted by the luminous rays are 

 distiibuted, M. Desains has verified the existence of pencils of heat distril)uted like 

 those of light. Moreover, by causing obscure heat, proceeding from a solar pencil 

 which has passed through M. Tyndall's solution of iodine in bisulphide of carbon, 

 to fall on the rays, M. Desains has observed that the intervals without heat 

 are greater than the obscure intervals given by the red light. Remark now the 

 gradation : the violet is more refrangil)le than the red, the red more refrangible 

 than the obsciu'e heat, consequent^' the magnitude of the intervals destitute of 

 rays varies in inverse proportion to the refrangibility. 



Several other experiments in optics have been transferred by M. Desains to 

 the domain of radiant heat. 1 will cite one of those n.'lating to the })olarization 

 of obscure heat. The rays of heat employed issue from a common oil lamp ; a 

 lens of glass collects them and causes them to converge on a prism of Iceland 

 spar achromatized. Two pencils issue from this spar: that which is called the 

 ordinary pencil encounters a second yrism of spar like the first; it is bilurcated 

 in its turn. Of the two pencils thus obtained, that which is called or(Jinari/-()nli- 

 «ar^ falls on a lens and converges towards the thermo-electric l)attery. AVhen 

 the principal sections of the two spars are perpendicular, neither light nor heat 

 arrive at the battery ; the ordinary-ordinarjf pencil is said to be extinguished. If 

 one of the spars be now made to turn upon itself, light and heat innnediately appear 

 in this pencil. Let us now i)lace between the two spars a trough, containing 

 Tyndall's solution ; the luminous rays are arrested, and beyond the trough there 

 remains oidy the obsciue heat. The destruction of this heat is complete when 



* From the Revue des coiirs scientijiqucs dc la Frunrr. et de I' 6t ranger, "Associution 

 Scientitique de France, (conferences de la Sorbounc,") ld07. 



