38 



SIX LECTURES ON LIGHT 



while magnesium ribbon bursts into vivid 

 combustion A sheet of platinized platinum 

 placed at the focus is heated to whiteness. 

 Looked at through a prism, the white-hot 

 platinum yields all the colors of the spectrum. 

 Before impinging upon the platinum, the 

 waves were of too slow recurrence to awaken 

 vision; by the atoms of the platinum, these 

 long and sluggish waves are in part broken 

 up into shorter ones, being thus brought 

 within the visual range. At the other end of 

 the spectrum, Stokes, by the interposition of 

 suitable substances, lowered the refrangibil- 

 ity so as to render the non- visual rays visual, 

 and to this change he gave the name of 

 Fluorescence.. Here, by the intervention of 

 the platinum, the refrangibility is raised, so 

 as to render the non-visual visual, and to 

 this change we give the name of Calorcs- 

 cence. 



At the perfectly invisible focus where these 

 effects are produced, the air may be as cold 

 as ice. Air, as already stated, does not ab- 

 sorb the radiant heat, and is therefore not 

 warmed by it. Place at the focus the most 

 sensitive air-thermometer : it is not affected 

 by the heat. Nothing could more forcibly 

 illustrate the isolation, if I may use the term, 

 of the luminiferous ether from the air. The 

 \vave-motion of the one is heaped up, without 

 sensible effect, upon the other. I may add 

 that, with suitable precautions, the eye may 

 be placed in a focus competent to heat plati- 

 num to vivid redness, without experiencing 

 any damage, or the slightest sensation either 

 of light or heat. 



These ultra-red rays play a most important 

 part in Nature. I remove the iodine filter, 

 and concentrate the total beam. A test-tube 

 containing water is placed at the focus : it 

 immediately begins to sputter, and in a min- 

 ute or two it boils. What boils it? Placing 

 the alum solution in front of the lamp, the 

 boiling instantly ceases. Now, the alum is 

 pervious to all the luminous rays; hence it 

 cannot be these rays that caused the boiling. 

 I now introduce the iodine, and remove the 

 alum ; vigorous ebullition immediately re- 

 commences. So that we here fix upon the 

 invisible ultra-red rays the heating of the wa- 

 ter. We are enabled now to understand the 

 momentous part played by these rays in Na- 

 ture. It is to them that we owe the warming 

 and the consequent evaporation of the tropi- 

 cal ocean ; it is to them, therefore, that we 

 owe our rains and snows. They are 

 absorbed close to the surface of the 

 ooean, and warm the superficial water 

 while the luminous rays plunge to 

 great depths without producing any 

 sensible effect. Further, here is a large flask 

 containing a freezing mixture. The aqueous 

 vapor of the air has been condensed and 

 frozen on the flask, which is now covered 

 with a white fur. Introducing the alum-cell, 

 we place the coating of hoar-frost at the in- 

 tensely luminous focus ; not a spicula of the 



frost is melted. Introducing the iodine-cell, 

 j and removing the alum, a broad space of the 

 frozen coating is instantly removed. Hence 

 we infer that the ice which feeds the Rhone, 

 the Rhine, and cthei rivers which have 

 glaciers for their sources, is released from its 

 imprisonment upon the mountains by the 

 invisible ultra- red rays of the sun. 



The growth of science is organic. The 

 end of to-day becomes to-morrow the means 

 to a remoter end. Every new discovery is 

 immediately made the basis of other discov- 

 eries, or of new methods of investigation. 

 About fifty years ago, CErsted, of Copen- 

 hagen, discovered the deflection of a mag- 

 netic needle by an electric current ; and 

 Thomas Seebeck, of Berlin, discovered that 

 electric currents might be derived from heat. 

 Soon afterwards these discoveries were turned 

 to account by Nobili and Melloni in the con- 

 struction of an apparatus which has vastly 

 augmented our knowledge of radiant heat. 

 The instrument is here. It is called a thermo- 

 electric pile; and it consists of thin bars of 

 bismuth and antimony soldered together in 

 pairs at their ends, but separated from each 

 other elswhere. From the ends of this ' 'pile" 

 wires pass to a coil of covered wire, within 

 and above which are suspended two magnetic 

 needles joined to a rigid system, and carefully 

 defended from currents of air. The heat, 

 then, acting on the pile, produces an electric 

 current ; t'he current, passing through the 

 coil, deflects the needles, and the magnitude 

 of the deflection may be made a measure of 

 the heat. The upper needle moves over a 

 graduated dial far too small to be seen. It 

 is now, however, strongly illuminated. Above 

 it is a lens which, if permitted, would form 

 an image of the needle and dial upon the 

 ceiling, where, however, it could not be con- 

 veniently seen. The beam is therefore re- 

 ceived upon a looking-glass, placed at the 

 proper angle, which throws the image upon 

 the screen. In this way the motions of this 

 small needle may be made visible to you all. 



The delicacy of this instrument is such 

 that in a room like this it is exceedingly dif- 

 ficult to work with it. My assistant stands 

 several ieet off. I turn the pile towards him: 

 the heat from his face, even at this distance, 

 produces a deflection of 90. I turn the in- 

 strument towards a distant wall, which I 

 udge to be a little below the average temper- 

 ature of the room. The needle descends and 

 passes to the other side of zero, declaring by 

 Lhis negative deflection that the pile feels the 

 chill of the wall . Possessed of this instrument, 

 of our ray-filter, and of our large Nicol prisms, 

 we are in a condition to investigate a subject 

 of great philosophical interest, and which 

 'ong engaged the attention of some of our 

 foremost scientific workers, Forbes being the 

 first successful one the substantial identity 

 of light and radiant heat. 



That they are identical in all respects can- 

 not of course be the case, for if they vr re 



