ACTINISM 



fact, that three very distinct sets of physical phenomena were manifested: Light, Heat, 

 and Chemical action. Chaptal, Berthollet, and Biot reported on this paper by M. Berard ; 

 and, as showing the extent to which this very important inquiry had proceeded in 

 the hands of this philosopher, the following quotation is given from their report : 



' M. Berard found that the chemical intensity was greatest at the violet end of the 

 spectrum, and that it extended, as Hitter and Wollaston had observed, a little 

 beyond that extremity. When he left substances exposed for a certain time to the 

 action of each ray, he observed sensible effects, though with an intensity continually 

 decreasing, in the indigo and blue rays. Hence we must consider it as extremely 

 probable, that if he had been able to employ reactions still more sensible, he would 

 have observed analogous effects, but still more feeble, even in the other rays. To 

 show clearly the great disproportion which exists in this respect between the energies 

 of different rays, M. Berard concentrated, by means of a lens, all that part of the 

 spectrum which extends from the green to the extreme violet, and ho concentrated, by 

 another lens, all that portion which extends from the green to the extremity of the red 

 ray. This last pencil formed a white point, so brilliant that the eyes were scarcely 

 able to endure it, yet the muriate of silver remained exposed more than two hours to 

 this brilliant point of light, without undergoing any sensible alteration. On the other 

 hand, when exposed to the other pencil, which was much less bright, and less hot, it 



was blackened in less than six minutes If we wish to consider solar light as 



composed of three distinct substances, one which occasions light, another heat, and the 

 third chemical combinations, it will follow that each of those substances 

 is separable by the prism into an infinity of different modifications, like 15 



Light itself; since we find, by experiment, that each of the three pro- 

 perties, chemical, calorific, and colourijic, is spread, though unequally, 

 over a certain extent of the spectrum. Hence we must suppose, on 

 that hypothesis, that there exists three spec- 

 trums, one above another ; namely, a calorific, 

 a colourific, and a chemical spectrum." 



This was the earliest indication of the pro- 

 bable existence of a physical influence, in the 

 solar rays, distinct from Light and Heat. A 

 large number of philosophers still hold to the 

 idea that the chemical changes produced by the 

 sunbeam are due to light, and this idea is con- 

 firmed in the public mind by the universal 

 adoption of the term photography (light- 

 drawings) to indicate the production of 

 pictures by the agency of the sunbeam. See 

 PHOTOGRAPHY. 



The actual conditions of the sunbeam will be 

 understood by reference to the annexed wood- 

 cut, fig. 14, and attention to the following de- 

 scription : a b represents the prismatic spectrum 

 as obtained by the decomposition of white 

 light by the prism or Newtonian luminous 

 spectrum, consisting of certain bands of colour. 

 Newton determined those rays to be seven in 

 number; red, orange, yellow, green, blue, indigo, 

 and violet; recent researches, by Sir John 

 Herschel and others, have proved the existence 

 of two other rays ; one, the extreme red or 

 crimson ray c, found at the least refrangible end of the spectrum, the other occurring 

 at the most frangible end, or beyond the violet rays, which is a lavender or grey ray. 

 Beyond this point up to/, Professor Stokes has discovered a new set of rays, which are 

 only brought into view when the light is received upon the surfaces of bodies which 

 possess the property of altering the refrangibility of the rays. Those rays have been 

 called the fluorescent rays, from the circumstance that some of the varieties of Fluor 

 Spar exhibit this phenomenon in a remarkable manner. (See FLUORESCENCE.) The 

 curved line L from a to c indicates the full extent of the luminous spectrum, the point 

 marked i. showing the maximum of illuminating power, which exists in the yellow ray. 

 Sir William Herschel and Sir Henry Englefield determined, in the first instance, 

 the maximum point for the calorific rays, and Sir John Herschel subsequently confirmed 

 their results, proving that the greatest heat was found below the red ray, and that it 

 gradually diminished in power with the increase of refrangibility in the rays, ceasing 

 entirely in the violet ray. Heat rays have been detected down to the point d, and 

 the curved line H indicates the extent of their action. 



