110 Dr. Draper's Further considerations 



From this table, 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 

 phaenomena can only be given by assuming the existence and 

 presence of another agent besides the light, and to which agent 

 the chemical effect is due; that media are known, which in 

 their absorptive action bear relations which are totally differ- 

 ent for these two agents ; and, finally, that, as prismatic ana- 

 lysis 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. 



In addition to the results obtained from the foregoing quan- 

 titative experiments, there are 'other phaenomena of a very 

 novel and interesting kind, from which we may draw an argu- 

 ment of overwhelming force. The discussion of these I shall 

 now take up. 



Early during the last century the remarkable appearance 

 of phosphorescence, excited in the Bolognean stone and cal- 

 cined oyster-shells, attracted the attention of chemists. Dufay, 

 in France, wrote several papers upon it ; and the experiments 

 of Wilson in England are perhaps as fine a specimen of phi- 

 losophical investigation as those early times can furnish. 

 These results, which few are now acquainted with, deserve to 

 be republished. 



To Becquerel we are indebted for one of the most remark- 

 able discoveries in connexion with this subject. He found 

 that the rays of an electric spark, which had passed through 

 glass, no longer preserved the quality of exciting phospho- 

 rescence ; 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 discharging a Leyden phial a little distance 

 off. The sulphuret will glow as brilliantly on the part co- 

 vered by the quartz as on the uncovered spaces, but under 

 the glass it will remain dark. 



Nevertheless, this same sulphuret, carried into the sunshine, 

 phosphoresces powerfully under glass, apparently showing that 

 there is a difference between the phosphorogenic emanation of 

 the sun, for thus M. Becquerel terms it, and that of an elec- 

 tric spark. 



On this radiation, as it comes from the sun, M. Becquerel 

 has treated in his paper, of which a translation is given in 



