Prof. L. Moser on the so-called Calorotypes. 359 



As the question as to the identity of light and heat is now 

 frequently discussed, I will communicate a fact on this sub- 

 ject which in my opinion will prove decisive : it had escaped 

 me when writing the paper on the question of the identity 

 contained in these Annalen, vol. lviii. p. 105. Heat, as is well 

 known, is emitted from bodies to which it has been conveyed, 

 presupposing that it was not employed to produce a chemical 

 change in them: the light, on the contrary, which effected 

 that peculiar change on the surface c bodies which subse- 

 quently is best rendered perceptible i»y the condensation of 

 vapours, is not again radiated, and it must therefore be as- 

 sumed that it has become extinct with this action. I have 

 made numerous experiments to transfer the effect of the light 

 from one plate to the other; sometimes I took iodized silver 

 plates with a not yet perceptible image from the camera ob- 

 scura, placed them in contact for a short or long period with 

 other iodized plates, or with plates of other metals ; some- 

 times I employed for the same purpose metallic plates on 

 which the image produced by imperceptible light was in a far 

 more advanced state. As soon as I became acquainted with 

 the behaviour of the oils I tried them, and separated the two 

 plates employed in the experiment by a layer of oil ; in no 

 case have I succeeded in detecting even a trace of transfer of 

 an image on to another plate. The light which has produced 

 its effect radiates accordingly no more. 



Now what shall we say to the experiments of Dr. Draper, 

 in which this re-radiation is assumed as something self-evident, 

 and upon which indeed is principally founded a theory of ti- 

 thonic rays ? I may inform those philosophers who are not 

 acquainted with the investigations of Dr. Draper, that these 

 new kind of rays are nothing else than the chemical rays which 

 have often been said to have been discovered in the solar spec- 

 trum. Dr. Draper has just as much discovered them, as the 

 imperceptible and latent light; both of which species of light 

 he likewise lays claim to, without there having been the least 

 mention of them in his papers. But to keep to the subject, 

 it is impossible not to be astonished at a physicist who con- 

 ceives he has discovered a new force — the tithonic, and as- 

 serts that it radiates without even having made one single ex- 

 periment to prove this. Dr. Draper knows no more than 

 what every person who has been engaged in experiments on 

 light is aware of, that the image of an iodized silver plate, as 

 it comes from the camera obscura, disappears after a time, 

 and can no more be made to appear in the vapours of mercury. 

 Does it hence follow that the image radiates from the plate ? 

 This is so little the case, that, according to my experience, 



