20 LECTURES TO SCIENCE TEACHERS. 



when any one of these rays impinges upon a molecule of 

 matter, it meets with an opposition to its motion ; it may be 

 that the swing of the wave may be in accord, or nearly so, 

 with the natural swing of the molecule. In such a case we 

 might expect no change to occur in a compound molecule 

 though motion be imparted to it. But it may happen that 

 the beats of the two are of such a nature that a violent 

 internal battering and sifting, as it were, of the molecule will 

 take place. It may be so driven and shaken about, that in 

 order to arrive at some sort of harmonious motion with the 

 wave, it may throw off one of the atoms of which the 

 molecule is composed. It seems probable that to pro- 

 duce photographic sensitiveness in a compound there are two 

 requirements, first, that the molecule should be set in motion 

 by the wave, second, that the motion must be of such a 

 nature that a sifting of its component atoms takes place. 

 The silver iodide 1 molecule is apparently in the greatest 

 internal discord with the blue ray waves, and when they 

 fall upon it, it throws off an iodine atom. If then we can 

 obtain a molecule which can be caused to vibrate by the im- 

 pact of the red ray waves, and yet be shaken by them, we 

 might expect that such a one would throw off a something 

 to render itself more in tune with that wave. 



I give beneath a table of comparative weights of the 

 different molecules of silver chloride, bromide, and iodide. 



Ag 2 Cl 2 Ag 2 Br 2 A 2 I 2 



287 376 470 



By loading any of these silver compounds we ought to be 

 able to produce a corresponding change in the limit of internal 

 discord. Thus by loading a silver chloride molecule with a 

 dead weight equal to 89, or the difference between 376 and 

 287, we ought to cause its limit to be the same as the bromide. 

 Again, we have every reason to believe that by slightly 

 loading the molecule of the bromide we might make its limit 

 of internal discord to be at the A line, or below it ; or again, 

 we may expect that by getting a compound of silver which is 

 heavier than 470, the molecular weight of silver iodide, we 

 might obtain similar results. In all cases we must suppose that 

 there is some portion of the molecule that can readily be 



1 The absorption spectrum of silver iodide as well as of bromide 

 shows the greatest absorption to take place in the blue. 



