OTHER INVISIBLE PRINCIPLES IN THE SUNBEAM. 69 



with the blue light. Tithonic rays are present in a beam which has passed through 

 quadrosulphuret of lime, but chloride of silver cannot detect them. 



252. An accurate examination of the quantity of light and the quantity of tithonic 

 rays passing through given solutions has, however, restored this argument to its pristine 

 force. Thus, it is proved, that of different media which have been tried, some trans- 

 mit more of the luminous and some more of the tithonic rays (Ap., CH. XVIII.) ; that 

 the rate of transparency for one is often totally different from the rate of transparency 

 for the other ; and although we have not yet been so fortunate as to discover any given 

 medium which is opaque to one of these principles and transparent to the other, as has 

 been done in the case of light, their physical independence is just as certain as though 

 such a medium was known. 



253. After commenting in the preceding paragraph on the weak point in the 

 argument drawn from absorbent media, it ought not to be left without showing how 

 experiments of that class may be rigorously used as proofs of the correctness of the 

 views given in 1837. It only requires that the sensitive paper or surface used should 

 be of such a character as to be affected by all the tithonic rays in the spectrum, irre- 

 spective of their order of refrangibility or ideal coloration. Such a substance is the 

 bromide of silver. Sir J. HERSCHEL has used it with a similar object in view (Phil. 

 Trans., 1840, p. 38). Suppose, therefore, we allow a solar spectrum to fall upon such 

 a changeable surface, after having passed through a yellow absorbent medium, as the 

 bichromate of potassa. The eye at once informs us that very little of the yellow and 

 orange light is lost, but we must keep the surface for a long time exposed before the 

 yellow and orange tithonic rays will have produced such a change as they would have 

 produced in a few moments if the bichromate had not intervened. In this case the 

 experiment is a fair one, and the deduction it gives holds good, because bromide of 

 silver is easily decomposed by the tithonic yellow ray. The bichromate of potash, 

 therefore, transmits yellow and orange rays of LIGHT copiously, but it transmits the 

 corresponding yellow and orange tithonic ray to a far less extent. The two princi- 

 ples are therefore distinct. 



254. Tliat there is nothing unphilosophical in supposing that an invisible principle 

 such as that of which we are speaking should exist in solar light, is shown by the anal- 

 ogy of radiant heat, a principle equally invisible to our eyes, but of which the exist- 

 ence is palpable enough to our other organs of sense. In a dark room we are utterly 

 unable to see a vessel of hot water, but its calorific emanations are plain to the hand, 

 even at a considerable distance. In like manner, this analogy is supported by the re- 

 cent discovery of BECQUEREL. For a long time it has been known that there are cer- 

 tain bodies, such as calcined oyster shells, which shine in darkness after a brief expo- 

 sure to the light. A hundred years ago it was discovered that the transient light of an 

 electric spark is sufficient to awaken the dormant glow of these bodies. Now BEC- 

 QUEREL has shown, that to the rays which thus issue from an electric spark, and cause 

 this wonderful phenomenon, glass is opaque, that light can pass through glass, but the 

 phosphorescent rays cannot. They also are invisible to the eye. 



255. It will be seen, by referring to CH. XII. of the A p., that there are certain phe- 



