EECENT PROGRESS IN RELATION TO THE THEORY OF HEAT. 233 



of tlie prineiplo of the identity of lieat and li2,"lit ; it explains all observed pecu- 

 liarities of radiation, whether chemical, calorilic, or Iniuinous. 



If we would comprehend, for example, how a solution of sulphate of quinine is 

 luminous in a dark chamber, when it is placed in the ultra-violet region of the 

 solar spectrum? Imagine a scries f)f tuning forks of different magnitudes assem- 

 bled together in the same place, and a sound produced at a distance. Several 

 of the forks will be thrown into vibration, namely, those which arc capable of 

 rendering the harmonic sounds of the exciting sound. Sonorous waves, longer 

 than the incident wave, will })rocced from the forks which render grave har- 

 monic sounds : the exciting sound will have generated graver sounds. Such is tho 

 analogy of fluorescence. The radiations incident to waves too short to excite tho 

 retina generate in the sulpliate of (piinine longer wavtN?, which are capable of pro- 

 ducing the sensation of light. Inversely an obscine radiation of a wave too long 

 to be luminous may, by encountering certain bodies, occasion therein more ra])icl 

 ethereal vibrations, and generate shorter waves, which shall bo luminous. These 

 vibrations are analogous to those which correspond to the sharp harmonics in 

 the acoustical ex])eriment which I have adopted for exemplification. Here wo 

 have the image of that kind of calorilic and luminous phenomenon which M. 

 Tyndall has termed calorcscence. 



The ray of heat which penetrates into a body is absorbed therein either in 

 whole or in part. So long as the question concerns a solid or liquid body wo 

 feel no dou])t as to the exactness of this proposition ; it is the simplest expression 

 of observed facts. But when it relates to a gas or to vapor the absorption is 

 much more difficult to demonstrate. We owe to M. Tyndall, in England, and 

 to M. Magnus, in Germany, the experimental proof of the absorption of heat by 

 gases and the measure of that absorption. 



Tlie experiments of M. Pouillet on the solar heat long since taught us that the 

 atmosphere retained a considerable portion of the rays emanating from the sun 5 

 but which of the gaseous elements of the air exerts the greatest absorption 1 At 

 present some approximation has been made to the sohition of this important 

 question, and I shall attempt to show at what point it has arrived. 



To ascertain the absorption of heat by a gas, avc will take, like M. Tyndall, 

 a tube of plate-tin of the length of two metres, w'hich bears a tubulure in the 

 middle and a tubulure towards each end. A pencil of obscure heat traverses this 

 tube, and encounters the thermo-electric battery. The calorific effect indicated 

 by the galvanometer is due, in part, to the ra^'s which pass ijito the tube parallel 

 to its axis, in part to those which have undergone sundry reflections on the walls 

 of the tube; the tube contains at this time only common air, naturally humid. 

 We exhaust this air by the tubulure of the middle, by means of a pneumatic 

 machine ; as the tulio is open at both ends the atmospheric air enters freely, and 

 notwithstanding the removal of the strata of air, the needle of the galvanometer 

 remains at rest. If coal gas be now introduced through the terminal tubulures 

 the deviation of the needle diminishes, wliich shows tliat heat no longer traverses 

 the tube as freely as before. Arrest the introduction of tho gas by continuing 

 the action of the pistons of the pneumatic macliine, air replaces the gas, and tho 

 needle returns to its former deviation. From this experiment it is inferred that 

 coal-gas has an absorbing power superior to that of atmos|)heric air. 



]M. Tyndall varied this experiment by causing dry air to pass into the tube, 

 and observed an augmentation in the deviation of tho needle of the galvanometer. 

 From this he concludes that the absorbing power of humid air is greater than 

 that of dry air, and that aqueous vapor exerts on heat a considerable degree of 

 absorption. A great numberof experiments made by other methods has led him 

 to the same conclusion. On the other hand M. Magnus, in operating after M. 

 Tyndall's or by other methods, foinid that humid air acted very nearly as dry 

 air, and that any great difference was only niaiii tested when water exists in the 

 air in a vesicular statCj similar to the water of clouds. 



