PHYSICAL FORCES. 



677 



beyond tho red was hotter than, the colored 

 region. He hence concluded that beside the vis- 

 ible sunshine there is a vast outflow of perfectly 

 invisible rays proceeding from the sun, and 

 that, measured by their thermal power, the 

 energy of these invisible radiations is greater 

 than that of all the visible rays taken together. 

 Forty years later these results were modified 

 and extended by the son of the discoverer, Sir 

 John Herschel, by a different method of inquiry. 



The inquiry was taken up and pursued with 

 remarkable success by Melloni. He achieved 

 his admirable results first by the adoption of 

 the thermo-electric pile as an instrument of re- 

 search far more sensitive and perfect than the 

 most delicate thermometer ; and second, by 

 the discovery that rock-salt is in a super- 

 eminent degree pervious to the dark rays ; it 

 permits almost the whole of them to pass, and 

 was hence termed the glass of heat. By the 

 use of rock-salt prisms he showed that the 

 dark heating rays, which are known as the 

 ultra-red or Herschellic rays, formed an invisible 

 spectrum at least as long as the visible one. He 

 found, also, that the maximum of radiant ther- 

 mal power lies as far OA one side the red as the 

 green light of the spectrum on the other. 



Prof. Tyndall has pursued these researches at 

 intervals during the last ten years, employing 

 the thermo-electric pile and the spectrum of the 

 electric light, produced by using the electric 

 lamp of Duboscq. He finds that the augmenta- 

 tion of temperature beyond the red in the spec- 

 trum of the electric light is sudden and enor- 

 mous. Representing the thermal intensities by 

 lines of proportional lengths, and erecting these 

 lines as perpendiculars at the places to which 

 they correspond, when we pass beyond the 

 red these perpendiculars suddenly and greatly 

 increase in length, reach a maximum, and then 

 fall somewhat more suddenly on the opposite 

 side of the maximum. When the ends of these 

 perpendiculars are united, the curve beyond the 

 red representing the dark radiation, rises in a 

 steep and massive peak, which quite dwarfs by 

 its magnitude the radiation of the luminous 

 portions of the spectrum. Interposing suitable 

 substances in the path of the beam this peak 

 may be in part cut away. Water, in certain 

 thicknesses, does this very effectually. The va- 

 por of water would do the same, and this fact 

 enables us to account for the difference between 

 the distribution of heat in the solar and in the 

 electric spectrum. The peak in the spectrum 

 of the electric light is much higher than in the 

 solar spectrum ; the latter having, probably, 

 been cut down by the aqueous vapor of the 

 atmosphere. 



But how may these dark radiations be sepa- 

 rated from the others ? A portion of them may 

 obviously be isolated by interposing an opaque 

 screen so as to cut off the luminous portion of the 

 spectrum the dark remainder being still sub- 

 ject to reflection, refraction, and condensation. 

 The attention of scientific men, however, from 

 Sir W. Herschel to the present time, has been 



directed to the discovery of some medium which 

 should be opaque to light but transparent to heat 

 something which might strain or sift the ray of 

 its luminous elements, while the thermal stream 

 is allowed freely to pass. Dark-colored glasses 

 were much used for this purpose, and latterly a 

 perfectly black glass. 



Prof. Tyndall's researches on the relations of 

 radiant heat to various gases led him to the 

 discovery that elementary gases were in a high 

 degree transparent to the dark rays, while the 

 compound gases, on the other hand, were in a 

 great degree opaque to them. Starting with 

 this clue, Prof. Tyndall was led to inquire into 

 the behavior of various elements for this pur- 

 pose. He found that sulphur dissolved in bi- 

 sulphide of carbon, and also bromine, w r ere 

 eminently transparent to the ultra-red rays. 

 He finally tried a solution of iodine in bi-sul- 

 phide of carbon, and arrived at the extraordi- 

 nary result that a quantity of dissolved iodine 

 sufficiently opaque to cut off the light of the 

 mid-day sun was, within the limits of experi- 

 ment, absolutely transparent to invisible radiant 

 heat. 



Concentrating by a small glass mirror silvered 

 in front, the rays emitted by the carbon points 

 of the electric lamp, a convergent cone of 

 light is obtained. Interposing in the path of 

 this concentrated beam a cell containing the 

 opaque solution of iodine, the light of the cone 

 is utterly destroyed, while its invisible rays are 

 scarcely disturbed. These converge to a focus 

 at which though nothing can be seen, even in 

 the darkest room the following effects are pro- 

 duced : paper and wood are set on fire, cigars 

 lit, charcoal ignited, and a mixture of hydrogen 

 and oxygen exploded. Metals, as zinc and mag- 

 nesium, are burned ; and plates of charcoal or 

 of copper, silver, or platinum with tarnished 

 surfaces, are raised to incandescence. A plate 

 of platinized platinum in the invisible focus is 

 made white hot, and when looked at through a 

 prism, its light yields a brilliant-colored spec- 

 trum. In this case there is an immediate and 

 direct conversion of heat into light. 



Some thirteen years ago Prof. Stokes pub- 

 lished the important discovery that by the 

 agency of sulphate of quinine and various other 

 substances the dark ultra-violet or active rays 

 of the spectrum could be rendered visible. 

 These invisible rays of high refrangibility, fall- 

 ing upon a proper medium, are beh'eved to cause 

 the molecules of that medium to oscillate in 

 slower periods than those of the incident waves. 

 But in that experiment with heat there is a 

 reverse effect. The dark thermal undulations 

 consist of slower vibrations than the luminous. 

 When these, therefore, fall upon a plate of 

 platinum and raise it to incandescence, their 

 periods of vibration are changed. But while 

 in Prof. Stokes' experiment the invisible rays 

 became visible by lowering their refrangibility, 

 in Tyndall's experiment the dark rays became 

 visible by raising their refrangibility. To the first 

 Prof. Stokes has given the name Fluorescence, 



