A FOURTH IMPONDERABLE.. 205 



934. By resorting to the tithonometer, we obtain information equally distinct upon 

 the second point, that the preliminary tithonization is not a transient effect which at 

 once passes away, but is, on the contrary, a persistent change. 



935. I tithonized the chlorine and hydrogen contained in the instrument, and kept 

 it in the dark for ten hours. On exposure to the lamp-rays it moved after a few sec- 

 onds, showing, therefore, that the change which had been impressed on the chlorine 

 was not lost. In the former case, 600 seconds had elapsed before any movement was 



visible. 



936. When, however, we remember that the invisible images on Daguerreotype 

 plates, and even photographic impressions on surfaces of resin, and probably all other 

 similar changes, are slowly effaced, it would be premature to conclude that tithonized 

 chlorine does hot revert to its original condition, I have sometimes thought that there 

 were in several of my experiments indications that this was taking place, but would not 

 be understood to assert it positively. Whether it be so or not, one thing is certain, 

 that the taking on of this condition and the loss of it is a very different affair from 

 any transient exaltation of action due to a temporary elevation of temperature, or the 

 contrary effect produced by cooling. 



CHAPTER XVIII. 



FARTHER CONSIDERATIONS ON THE EXISTENCE OP A FOURTH IMPONDERABLE. 



CONTENTS : Defects of former Evidence. A new Photometer. Measures of the Light 

 transmitted by Coloured Solutions. Explosion of Chlorine and Hydrogen by a dis- 

 tant Electric Spark. Absorptive Action of Media. 



The Absorptive Action on Light and the Tithonic Rays follows different Laics. 



Opacity of Glass for Phosphoric Rays. Determination of the Refrangibility of the 

 Phosphoric Rays of an Electric Spark. Refrangibility of the same Rays in the Vol- 

 taic Arc of Flame. Professor Henry's Experiments. 



These Facts serve to prove that there are more than three Imponderables. 



(From the London, Edinburgh, and Dullin Philosophical Magazine for August, 1844.) 



. 937. IN the Philosophical Magazine for December, 1842, I brought forward several 

 facts which had caused me to form the opinion that the chemical rays of the older optical 

 writers constitute, in reality, a new imponderable substance, which should be placed in 

 the same rank with light, heat, and electricity. To the views then given, I propose, in 

 this communication to return again, and furnish farther proof of their correctness. An 

 extended examination, which has occupied me several years, has served to deepen my 

 conviction of the truth of this doctrine. 



933. Great changes in the fundamental theories of science ought not to be lightly 



