SIX LECTURES ON LIGHT. 



37 



dazzling btightness, but it is not even charred. 

 On removing the alum-cell, however, the 

 paper instantly inflames. There must, there- 

 fore, be something in this beam besides its 

 light. The light is not absorbed by the 

 white paper, and therefore does not burn the 

 paper ; but there is something over and above 

 the light which is absorbed and which pro- 

 vokes combustion. What is this something? 

 In the year iSoo Sir William Herschel 

 passed a thermometer through the various 

 colors of the solar spectrum, an I marked the 

 rise of temperature corresponding to each 

 color. He found the heating effect to aug- 

 ment from the violet to the red ; he did not, 

 however, stop at the red. but pushed his 

 thermometer into the dark space beyond it. 

 Here he found the temperature actually higher 

 than in any part of the visible spectrum. By 

 this important observation, he proved that 

 the sun emitted dark heat-rays which are en- 

 tirely unfit for the purposes cf vision. The 

 subject was subsequently taken up by See- 

 beck, Melloni, Miiller, and others, and within 

 the last few years it Ins been found capable 

 of unexpected expansions and applications. 

 A method has been devised whereby the solar 

 or electric beam can be so filtered as to detach 

 from it and preserve intact this invisible 

 ultr.-red emission, while the visible and ultra- 

 violet emissions are wholly iatercepted. We 

 are thus enabled to operate at will upon the 

 purely ultra-red waves. 



In the heating of so.id bodies to incandes- 

 cence this non-visual emission is the neces- 

 sary basis of the visual. A platinum wire is 

 stretched in front of the table, and through 

 it an electric current flows. It is warmed by 

 the current, and may be felt to be warm by 

 the hand ; it also emits waves of heat, but no 

 1 ght. Augmenting the strength of the cur- 

 rent, the wire becomes hotter ; it finally 

 glows with a sober red light- At this point 

 Dr. Draper many years ago began an inter- 

 esting investigation. He employed a vol- 

 taic current tD heat his platinum, and he 

 studied by means of a prism the successive 

 introduction of the colors of the spectrum. 

 His first color, as here, was red ; then came 

 orange, then yellow, then green, and lastly 

 all the shades of blue. Thus as the tempera- 

 ture of the platinum was gradually aug- 

 mented, the at^rns were caused to vibrate 

 more rapidly, shorter waves wer3 thus pro- 

 duced, until finally he obtained the waves 

 corresponding to the entire spectrum. As 

 each successive color was introduced, the 

 colors preceding it became more vivid. Now, 

 the vividness, or intensity of light, like that 

 of sound, depends, not upon the length of the 

 wave, but on the amplitude of the vibration. 

 Hence, as the red grew more intense as the 

 more refrangible colors were introduced, we 

 are forced to conclude that, side by side with 

 the introduction of the shorter waves, we had 

 an augmentation of the amplitude of the 

 longer ones. 



These remarks apply, not only to the vis- 

 ible emission examined by Dr. Draper, but 

 to the invisible emission which preceded 

 the appearance of any light. In the emis- 

 sion from the white-hot platinum wire now 

 before you the very waves exist with Avhich 

 we started, only their intensity has been in- 

 creased a thousand-fold by the augmentation 

 of temperature necessary to the production 

 of this white light, i oth effects are bound 

 together: in an incandescent solid, or in a 

 molten solid, you cannot have the shorter 

 waves without this intensification of the 

 longer ones. A sun is possible only on 

 these conditions; hence Sir William Her- 

 schel's discovery of the invisible ultra-red 

 solar emission. 



The invisible heat, emitted both by dark 

 bodies and by luminous ones, flies through 

 space with the velocity of light, and is called 

 radiant heat. Now, radiant heat may bj 

 made a subtle and powerful explorer of 

 molecular condition, and of late years it 

 has given a new significance to the art of 

 chemical combination. Take, for example, 

 the air we breathe. It is a mixture of oxygen 

 and nitrogen; and with regard to radiant 

 heat it behaves like a vacuum, being incom- 

 petent to absorb it in any sensible degree. 

 But permit the same two gases to unite 

 chemically; without any augmentation of the 

 quantity of matter, without altering the gase- 

 ous condition, without interfering in any 

 way with the transparency of the gis, the 

 act of cherr.ical uni n is accompanied by an 

 enormous diminution of its diathermancy, or 

 ^erviousness to radiant heat. The reseaiches 

 which established this result also proved the 

 elementary gases generally to be highly 

 :ransparent to radiant heat. This, again, 

 ed to the proof of the diathermancy of ele- 

 mentary liquids, like bromine, an t of solu- 

 tions of the elements sulphur, phosphorus, 

 and iodine. A spectrum is now before you, 

 and you notice that this transparent bisul- 

 Dhideof carbon has no effect upon the colors. 

 Dropping into the liquid a few flakes of 

 odme, you see the middle of the spectrum 

 cut away. By augmenting the quantity of 

 'odine, we invade the entire spectrum, and 

 inally cut it off altogether. Now, the iodine 

 ,vhich proves itself thus hostile to the light is 

 perfectly transparent to the ultra-red emis- 

 sion with which we have now to deal. It, 

 herefore, is to be our ray-filter. 



Placing the alum-cell again in front of the 

 electric lamp, we assure ourselves, as before, 

 of the utter inability of the concentrated 

 J ight to fire white paper. By introducing a 

 cell containing the solution of iodine, the 

 ight is entirely cut off. On remov- 

 ng the alum-cell, the paper at the dark focus 

 s im tantly set en fire. Black paper is more 

 bsorbent than white for these ultra-red rays; 

 and the consequence is, that with it the sud- 

 denness and vigor of the combustion are aug- 

 mented. Zinc is burnt up at the same place, 



