158 ANALOGIES BETWEEN THE CHEMICAL RAYS AND HEAT. 



sian government, found very long exposures in the camera needful to procure impres- 

 sions of the ruined monuments of the deserted cities existing there. This was not due 

 to any defect in his lens; it was a French achromatic, and I tried it in this city with 

 him before his departure. The proofs which he obtained, and which he did me the 

 favour to show me on his return, had a very remarkable aspect. More recently, in the 

 same country, other competent travellers have experienced like difficulties, and, as I 

 am informed, failed to get any impressions whatever. Are these difficulties due to the 

 antagonizing action of the negative rays upon the positive. 



CHAPTER XIII. 



ON A NEW IMPONDERABLE SUBSTANCE, AND ON A CLASS OF CHEMICAL RAYS ANALOGOUS 



TO THE RAYS OF DARK HEAT. 



(From the London, Edinburgh, and Dublin Philosophical Magazine for December, 1842.) 



CONTENTS : Analogies between the Chemical Rays and Heat. New Nomenclature pro- 

 posed. Tithonic Rays. Independence of Tithonic Rays and Light. Independence 

 of Tithonic Rays and Heat. Dark Tithonic Rays. 



651. IN chapter twelve I have pointed out several analogies which may be observed 

 between the phenomena of the chemical rays and those of radiant heat. 



652. In this memoir it is my intention to show still more striking points of analogy, 

 and also to direct the attention of chemists to equally striking points of discordance. 



653. It will be seen, from the remarkable facts detailed in this paper, that we are 

 now forced to recognise the existence of a new imponderable agent, analogous in many 

 of its properties to light, heat, and electricity, yet differing as much from them all as 

 they do from one another. 



654. So far as chemical analogies can direct us, there does not appear anything un- 

 philosophical in the supposition of the existence of many imponderable agents analo- 

 gous to those already known. The progress of science has indeed tended in different 

 directions in the cases of the imponderable and ponderable bodies. Among the former, 

 we have successively seen the agents that are concerned in galvanic phenomena and 

 those of magnetism merged into electricity ; but the ponderable bodies, especially those 

 of a metallic kind, have greatly increased in number, though, so far as their more obvi- 

 ous physical properties are concerned, the differences of many ar.e almost undistinguish- 

 able. We have thus found it necessary to invert the maxims of the early cultivators 

 of chemistry, who extended the number of ethereal agents very greatly, and believed 

 that all metals and other ponderable principles were modifications of one or two pri- 

 mordial and elementary forms. 



655. Centuries ago it was discovered that the sun's light had the property of effect- 

 ing chemical changes in bodies, and it is stated that SCHEELE first noticed that this 



