Prof. Tyndall's Contributions to Molecular Physics. 4>o7 



To what, then, are we to ascribe the deportment of iodine 

 towards luminous and obscure heat ? The difference between 

 both qualities of heat is simply one of period : in the one case 

 the waves which convey the energy are short and of rapid recur- 

 rence; in the other case they are long and of slow recurrence. 

 The former are intercepted by the iodine, and the latter are 

 allowed to pass. Why? There can, I think, be only one answer 

 to this question — that the intercepted waves are those whose 

 periods coincide with the periods of oscillation possible to the 

 atoms of the dissolved iodine. Supposing waves of any period 

 to impinge upon an assemblage of molecules of any other period, 

 it is, I think, physically certain that a tremor of greater or less 

 intensity will be set up among the molecules ; but for the motion 

 to accumulate so as to produce sensible absorption, coincidence 

 of period is necessary. Briefly defined, therefore, transparency 

 is synonymous with discord, while opacity is synonymous with 

 accord between the periods of the waves of ether and those of 

 the molecules of the body on which they impinge. The opacity, 

 then, of our solution of iodine to light shows that its atoms are 

 competent to vibrate in all periods which lie within the limits of 

 the visible spectrum ; while its transparency to the extra-red 

 undulations demonstrates the incompetency of its atoms to 

 vibrate in unison with the longer waves. 



This simple conception will, I think, be found sufficient to 

 conduct us with intellectual clearness through a multitude of 

 otherwise perplexing phenomena. It may of course be applied 

 immediately to that numerous class of bodies which are transpa- 

 rent to light, but opake in a greater or less degree to radiant 

 heat. Water, for example, is an eminent example of this class 

 of bodies : while it allows the luminous rays to pass with free- 

 dom, it is highly opake to all radiations emanating from obscure 

 sources. A layer of this substance one-twentieth of an inch* 

 thick is competent, as Melloni has shown, to intercept all rays 

 issuing from bodies heated under incandescence. Hence we 

 may infer that, throughout the range of the visible spectrum, 

 the periods of the water-molecules are in discord with those of 

 the ^ethereal waves, while beyond the red we have coincidence 

 between both. 



What is true of water is, of course, true in a less degree of 

 glass, alum, calcareous spar, and of all the substances named in 

 the first section of this paper. They are all in discord with the 

 visible spectrum ; they are all more or less in accord with the 

 extra-red undulations of the spectrum. 



Thus also as regards lampblack : the blackness of the sub- 

 stance is due to the accord which reigns between the oscillating 

 periods of its atoms and those of the waves embraced within the 



Phil. Mag. S. 4. Vol. 28. No. 191. Dec. 1864. 2 H 



