LAWS DEDUCED FROM IT. 7 



mate of potash what has been already proved for the chrysotype preparation that the 

 active rays are absorbed, and that the inactive rays escape. 



347. Without dwelling longer on the detail of farther instances, it appears that the 

 general laws under which these phenomena take place are as follows : 



1st. When a ray impinges on a sensitive surface, or passes through a changeable me- 

 dium, with the chemical effect that takes place, the constitution of the ray is corre- 

 spondingly disturbed. A change in the composition of the medium involves a change 

 in the ray. 



2d. Rays which thus disappear by absorption are occupied in disturbing the consti- 

 tution of the ponderable medium. 



3d. Rays which are inactive, or which are not involved in the chemical change go- 

 ing on, escape from the medium by being transmitted or reflected. 



348. The definite views which we thus gather respecting the absorption of the dif- 

 ferent constituents of the solar rays, and the production of chemical changes, lead us 

 by very simple steps to regard one as the cause and the other as the effect. In cases 

 like these, the safest way to true conclusions is, to be guided by analogies. It is true, 

 that the properties of the different agents in the solar beams are sufficiently distinct, 

 but, as radiant principles, they have certain qualities in common. The heat of a sun- 

 beam converged by a lens on red oxide of lead is absorbed by that substance ; oxygen 

 gas is given off, and a lemon-coloured protoxide remains behind ; no farther absorp- 

 tion of heat now takes place, and no farther chemical changes ensue. Heat, therefore, 

 as well as light, or the tithonic or phosphoric rays, in producing its effects, undergoes 

 absorption. 



349. In thus making the phenomenon of absorption the fundamental fact of our the- 

 ories on the chemical action of the sunbeam, and in giving a distinct prominence to it, 

 a great deal of precision will be brought into our theoretical discussions. Almost every 

 experimenter has, to a certain extent, recognised the truth of these views in a general 

 way, though without clearly setting forth the exact conditions under which absorption 

 takes place. In 1841 (Ap., CH. XII.) I published some experiments to show how 

 completely all the chemical changes effected by light were under the control of ab- 

 sorption, and that the sensitiveness of any given changeable compound could be altered 

 by altering its optical constitution. 



350. So, in the case of radiant heat, a piece of polished silver exposed to the focus 

 of the most powerful burning mirror never melts, not because it is an infusible body, 

 but because such an optical constitution has been given to it, that it reflects the heat 

 which impinges on it. If the polish be taken off, and the surface slightly roughened, 

 it melts in an instant, because it can absorb the rays. So, too, different coloured pieces 

 of cloth, exposed to the sunshine upon snow, will sink to different depths, because the 

 quality of coloration which they possess enables them to absorb the heat more or less 

 rapidly, and the calorific effect is determined by the optical constitution. 



351. Apparently nothing can be of a more irregular character than the photographic 

 impressions and points of maxima left by the solar spectrum on various surfaces. With 

 trivial causes the position and dimensions of those impressions change, but when we 



