PRODUCTION OF SPECTRAL APPEARANCES. 165 



precisely, in the case of heat. Much of the heat of the sun passes through plate glass, 

 and if it falls on a dark surface that can absorb it, that surface becomes presently warm 

 and commences radiating; but the physical constitution of these rays is changed : they 

 cannot get through the glass ; and if a non-conducting black surface, half covered by a 

 piece of glass and half in the free air, were exposed to the sun, the covered half would, 

 for these obvious reasons, become the hotter. For the same reason, precisely, in the 

 tithonic experiment, the glass increases the final effect by obstructing radiation. 



706. It is very obvious why such effects cannot be produced on gloom}' days. If 

 at such times we were to expose a piece of black cloth, partially covered by glass, no 

 difference of temperature would be perceptible in its covered and uncovered portions. 

 The reasons are analogous in each case. 



707. An experiment, the same in principle as Sir JOHN HERSCHEL'S, may be easily 

 made. Upon a sensitive plate that has been exposed a short time to a feeble light, 

 place a convex lens ; the arrangement being left for a time in a dark room. When 

 you have mercurialized, you will find a central dark point corresponding with the point 

 of contact, and round it a white areola that shades gradually and imperceptibly away. 

 With a lens with which I have occasionally made this experiment, the areola is nearly 

 an inch in diameter, the lens being a double convex of about two inches focus. 



Note added to the preceding Chapter. 



ON THE RAPID DETITHONIZING POWER OF CERTAIN GASES AND VAPOURS, AND ON AN 

 INSTANTANEOUS MEANS OF PRODUCING SPECTRAL APPEARANCES. 



(From the London, Edinburgh and Dullin Pkiltuopkual Magazine for Mtrck, 1843.) 



708. FOR some time after I was acquainted with the phenomena mentioned in the 

 last chapter, and there referred to radiation, I was led to attribute them to a peculiar 

 properly which certain gases and vapours possess, of which I propose now to give a 

 detailed description. 



709. This property is a power of effecting a very rapid detithonization of surfaces 

 *hat have been powerfully tithonized. 



710. It affords the means of instantly procuring spectral appearances of external 

 forms. 



711. Referring now, in the first place, to the analogies of caloric : a body which has 

 been warmed cools down to a temperature that is in equilibrium with that of objects 

 around in several different ways, by radiation, by currents in the air, and often by direct 

 conduction, each of these tending to produce the same result 



712. A sensitive surface, which has been disturbed by exposure to the daylight or 

 lamplight, has the quality of restoring itself to its primitive condition when kept in the 

 dark. DAGUF.RRE noticed this in the case of certain resinous bodies ; other experi- 

 menters have likewise proved that it takes place with some varieties of the ordinary 

 photogenic preparations. I have found that it holds in the coloured films on the sur- 

 face of silver. 



