PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
343 
obtained entirely separated from the luminous one. By means of a prism of the trans- 
parent bisulphide, I determined the position of the spectrum of the electric light upon 
a screen, and behind the screen placed a thermo-electric pile so that when the screen 
was removed the extra-red rays fell upon the pile. I then substituted an opaque prism 
for the transparent one : there was no visible spectrum on the screen ; but the removal 
of the latter at once demonstrated the existence of an invisible spectrum by the thermo- 
electric current which is generated, and which was powerful enough to dash violently 
aside the needles of a large lecture-room galvanometer. 
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 recurrence ; 
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. Sup- 
posing 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, trans- 
parency 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 intel- 
lectual clearness through a multitude of otherwise perplexing phenomena. It may of 
course be applied immediately to that numerous class of bodies which are transparent 
to light, but opaque 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 freedom, it is highly opaque 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. 
MDCCCLXIV. 3 A 
