o?i the Existence of a Fourth Imponderable. 115 



the characters of the phosphorogenic emanation to do with 

 the existence of the tithonic rays as a fourth imponderable? 

 A few words will show. From these considerations and ex- 

 periments we have arrived at the conclusion, that there exist 

 in the beams of an electric spark invisible rays.^ which are there- 

 fore totally distinct from light. They occupy the same spec- 

 trum region as the tithonic rays which decompose iodide of 

 silver; their leading character is, that glass which is transpa- 

 rent to the rays of light is opake to them. 



But the admission of this fact breaks down at once the doc- 

 trine of a trinity of imponderables, and compels us to enlarge 

 our list of those living forces of chemistry. The great ob- 

 stacle which is in the way of admitting the tithonic rays as a 

 fourth imponderable, is in the circumstance that it would im- 

 press a very serious change on that science, and apparently 

 afford an argument of weight against the mathematical theory 

 of light. I believe that some great generalization will here- 

 after prove that all these imponderables are modifications of 

 one primordial principle. I also believe that some capital ex- 

 periment will hereaft^ show that the forty different metals we 

 are acquainted with are merely modifications of one or two 

 more simple forms ; but these are things that we are unable 

 to deal with now ; and viewing the experiments which have 

 been made in the last few years, not as mathematicians, but as 

 chemists, all men must acknowledge that our prevailing doc- 

 trines of the nature and number of the imponderables are 

 liable, before long, to undergo a very serious modification. 



The admission that the phosphorogenic emanation and light 

 are principles difTering from the tithonic rays and from each 

 other, relieves us of much difficulty in increasing our list of 

 imponderables. If these principles differ thus intrinsically 

 from one another, the question comes home to us — what are 

 they? If electricity and heat and light are three recognised 

 imponderables, are not the tithonic rays a fourth, and the phos- 

 phorogenic ematiation ajifth ? 



In view of this, I would suggest the propriety of ceasing to 

 call these last by the epithet o^i emanations^ and of giving them 

 the more appropriate name of phosphorogenic rays. 



And now what appears to become of M. Becquerel's hypo- 

 thesis, that all the different effects we have been consider- 

 ing are due to light, and are presented to us under different 

 aspects because everything depends on the nature of the recei- 

 ving surface ; that it is the same principle which affects the eye 

 as light, decomposes chloride of silver as a tithonic ray, and 

 makes sulphuret of lime shine as a phosphorogenic ray ; that 

 the difference is not in the radiant principle, but in the surface 



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