ABSORPTION IN THE TWO CASES FOLLOWS DIFFERENT LAWS. 



211 



that the absorptive action of naphtha, of a slightly-yellow tint, is very much greater for 

 the tithonic rays than for the luminous rays. 



967. Consequently, it follows that it is not the light comprehended between the ex- 

 treme blue and extreme violet rays which brings about the union of chlorine and hydro- 

 gen, but another aud invisible class of rays, which is absorbed by naphtha under a differ- 

 ent law for that for its action on light. 



968. The foregoing example shows the mode by which I have obtained the results 

 of the following table : 



TABLE SHOWING THE ABSORPTIVE ACTION OF CERTAIN MEDIA FOR THE 

 LUMINOUS AND TITHONIC RAYS. 



From this tabk, therefore, we gather that the chemical effect produced by a given ray 

 has no relation to the quantity of light which is in it; that a satisfactory explanation 

 of the phenomena can only be given by assuming the existence and presence of an- 

 other agent besides the light, and to which agent the chemical effect is due; that me- 

 dia are known which in their absorptive action bear relations which are totally differ- 

 ent for these two agents ; and, finally, that, as prismatic analysis has also previously 

 shown, no explanation can be given of these results by imputing them to the agency 

 of light, we are forced to admit the existence of another imponderable principle, the 

 same as that which passes in these papers under the name of tithonic rays. 



969. In addition to the results obtained from the foregoing quantitative experiments, 

 there are other phenomena of a very novel and interesting kind, from which we may 

 draw an argument of overwhelming force. The discussion of these I shall now take up. 



970. Early during the last century, the remarkable appearance of phosphorescence 

 excited in the Bolognean stone and calcined oyster-shells, attracted the attention of chem- 

 ists. DUFAY, in France, wrote several papers upon it; and the experiments of WILSON 

 in England are, perhaps, as fine a specimen of philosophical investigation as those early 

 times can furnish. These results, which few are now acquainted with, deserve to be 

 republished. 



971. To BECQUEREL we are indebted for one of the most remarkable discoveries in 

 connexion with this subject He found that the rays of an electric spark which had pass- 

 ed through glass no longer preserved the quality of exciting phosphorescence, but 

 when they passed through quartz they retained that power unimpaired. This result 

 is very strikingly shown by placing a piece of colourless glass and a piece of quartz on 

 a surface covered with sulphuret of lime (oyster-shells calcined with sulphur), and dis- 

 charging a Leyden vial a little distance off. The sulphuret will glow as brilliantly 

 on the part covered by the quartz as on the uncovered spaces, but under the glass it 

 wil. remain dark. 



