288 Sir William B. Grove [April 20, 



tLroTigliout the interplanetary spaces and tliroughout the universe so 

 far as we know it. Some suppose this ether to be of a specific 

 character differing from that of ordinary gases, others that it is in the 

 nature of a highly attenuated gas ; but, whatever it be, it cannot be 

 affected by undulations or vibrations without being moved, and when 

 matter is moved by any force it must offer resistance to that force, 

 and hence we get antagonism between force and resistance. Light 

 also takes time in overcoming this resistance, i. e. in pushing aside 

 the ether. It travels, no doubt, at a good pace — about 190,000 miles 

 in a second ; but even at this rate, and without being particular as to 

 a few millions of miles, it takes three years and a quarter to reach us 

 from the star which, so far as we know, is the nearest to us, viz. 

 a Centauri. The ether, or whatever it may be called, tenuous as it is, 

 is not unimportant, though it be not heavy. Without it we should 

 have no light and possibly no heat, and the consequences of its 

 absence would be rather formidable. I believe you have heard Dr. 

 Tyndall on this subject. Supposing the visible universe to be as it 

 is now supjiosed to be, i. e. in no part a mere vacuum, there can be no 

 force without resistance in any part of it. 



But photography carries us further, it shows us that light acts 

 on matter chemically, that it is capable of decomposing or forcing 

 asunder the constituents of chemical compounds, and is therefore a 

 force met by resistance. In the year 185G I made some experiments, 

 published in the ' Philosophical Magazine ' for January 1857, which 

 seemed to me to carry still further what I may call the molecular 

 fight between light and chemical affinity, and among them the follow- 

 ing. Letters cut out of jDaper are placed between two polished squares 

 of glass with tin-foil on the outsidcs. It is then* electrized like a 

 Leyden jar, for a few seconds, the glasses separated, the letters blown 

 off", and the inside of one of the glasses covered with i^hotograi)hic 

 collodion. This is then exposed to diffusp daylight, and on being 

 immersed in the nitrate of silver bath the part which had been covered 

 with the paper comes out dark, the remainder of the plate being un- 

 affected. (This result was shown by the electric light lantern.) Ih 

 this case we see that another imponderable force, electricity, invisibly 

 affects the surface of glass in such a way that it conveys to another 

 substance of definite thickness, viz. the prepared collodion, a change 

 in the chemical relations of the substance (iodide of silver) pervading 

 it, enabling it to resist that decomposition by light which but for some 

 unseen modification of the surface of the glass plate it would have 

 undergone, and no doubt the force of light being unable to effect its 

 object was reflected or dispersed, and instead of changing its mode of 

 motion in effecting chemical decomposition, it goes off on other 

 business. The visible effect is in the collodion film alone. I have 

 stripped that off, and the imprint remains on it, the surface of the 

 glass being, so far as I could ascertain, unaffected. Thus in the film 

 over the protected part, light conquers chemical affinity ; in that over 

 the non-protected part, chemical af&nity resists and conquers light, 



