Class of Chemical Rays analogous to the Rays of Dark Heat. 46 1 



servable also with many other tithonographic compounds. 

 Transparent minerals, such as topaz, selenite, Iceland spar, 

 quartz, produced the same results as glass. But on gloomy 

 days the phsenomena did not appear, a bright sunshine being 

 apparently requisite for their production. " When a piece of 

 nitrated paper, for instance, was rolled round a cylindrical 

 surface of moderate convexity, covered with black velvet, and 

 the piece of glass laid gently in contact with it, the effect of 

 sunshine was exalted at the line of contact, but on either side 

 of that line as the interval increased the influence of the glass 

 diminished, and at less than half an inch distance no difference 

 could be perceived between the impressions under the glass 

 and in the free air." 



Now all this is precisely what should happen if the tithono- 

 graphic compound radiates whilst it is undergoing decompo- 

 sition. The rays, which come from the sun, pass through the 

 glass with but little loss from absorption, falling upon the ni- 

 trate they decompose it, and now it commences radiating, but 

 the physical character of these rays is very different from the 

 character they possessed before impinging on the nitrate. Now 

 they cannot get through the glass, before they passed without 

 difficulty. So it is 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. 



It is very obvious why such effects cannot be produced on 

 gloomy days. If at such times we were to expose a piece of 

 black cloth, partially covered by glass, no difference of tem- 

 perature would be perceptible in its covered and uncovered 

 portions. The reasons are analogous in each case. 



An experiment the same in principle as Sir John HerschePs 

 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. 



