100 SPECIFIC ABSORPTION. INSTRUMENTAL ARRANGEMENTS. 



of three simpler rays ; a ray endued with chemical energy, and of a similar composi- 

 tion to the former, as analogy would lead us to suspect. Again, each of these element- 

 ary rays is composed of particles, one half of which have their planes of polarization 

 at right angles to the other. All these elements taken together constitute a beam 

 of the sun's light. Emanations from the sun, after they have undergone the absorptive 

 action of the atmosphere of that great luminary, and of that of the earth, still reach us in 

 abundance, accompanying his light, and traversing the great vacuum, perhaps as far as 

 his attraction is felt. 



386. If we take a coloured medium of any kind, and transmit through it a beam of 

 the sun's light, we find, on examination, that certain of the rays exciting vision are 

 absorbed, that the light which passes through is not homogeneous, for it is capable 

 of decomposition by the prism ; it is a compound coloured ray, consisting of all the 

 rays, complementary to those which the medium has absorbed. Nor is this absorbing 

 effect confined to the rays producing vision; the rays of heat suffer in like manner; some- 

 times those which are more refrangible are wanting, sometimes those which are of less, 

 or of medium refrangibility, are absent. Often, at the same time, do the chemical rays 

 sustain a similar attack. There are solutions and media transparent to light, and 

 nearly opaque to heat ; there are others transparent to light and to heat, and opaque to 

 the chemical ray. It is from these facts that we are able to establish the separate 

 existence of three genera of rays in the sunbeam, each of which is essentially distinct 

 in its properties, and different in its mode of action from the others. Our eye can de- 

 tect, in the rays exciting vision, difference of constitution, because we are able to per- 

 ceive a difference of colour. Had we specific organs for indicating difference in the 

 heat-making, or chemical rays, perhaps we might find in them a similar constitution. 



387. It is between three and four years since that the investigation which forms the 

 subject of this memoir was first commenced, under the form of an examination of the 

 properties of the chemical ray. In the Journal of the Franklin Institute some of the 

 earlier results are recorded, and among them the fact, that the crystallization of cam- 

 phor, which has long been known to take place on the enlightened sides of vessels 

 exposed to the sun, occurs with very great rapidity, if the glass in which it is tried be 

 exhausted of air. In tracing out this fact, to ascertain its cause, a field of no common 

 interest has been entered. I do not here present a record of the facts as they were 

 successively developed by an analysis of the phenomena, but place them in that order 

 which appears to me the best to obtain a true estimate of their bearing. 



388. Into a darkened chamber, the shutter of which is seen in section at a a, Jig. 39, 

 a beam of the sun's light may be made to pass horizontally, by means of a mirror 

 of silvered glass, c. The mirror which I use is one belonging to a solar microscope, 

 and by turning the milled screws, e e, it can be brought into any position required to 

 throw a beam horizontally into the room, no matter what may be the place of the sun. 

 A brass tube, /, belonging to the same instrument, and two inches in diameter, can 

 be screwed into the position figured, if desirable ; there is also a lens, g, which may oc- 

 casionally be fixed at g; its focus is nine inches, its diameter about two inches, and the 

 diameter of the sun's image T V of an inch. 



