ELECTRICITY. 87 



changes. One of the plates of glass having been electrified 

 in the manner just mentioned, is coated, on the side impressed 

 with the invisible electrical image, with a film of iodised 

 collodion in the manner usually adopted for photographic 

 purposes ; it is then in a dark room immersed in a solution 

 of nitrate of silver ; then exposed to diffuse light for a few 

 seconds. On pouring over the collodion the usual solution 

 of pyrogallic acid, the invisible electrical image is brought 

 out as a dark device on a light ground, and can be permanently 

 fixed by hyposulphite of soda. The point worthy of obser- 

 vation in this experiment is, that this permanent image exists 

 in the collodion film, which can be stripped off the glass, dried, 

 and placed on any other surface, so that the molecular change 

 consequent on electrisation has communicated, by contact or 

 close proximity, a change to the film of collodion corres- 

 ponding in form with that on the glass, but being undoubtedly 

 of a chemical nature. Electricity has, moreover, in this ex- 

 periment so modified the surface of glass, that it can, in its 

 turn, modify the structure of another substance so as to alter 

 the relation of the latter to light. It would require a curious 

 complication of hypothetic fluids to explain this ; but if elec- 

 tricity and light be supposed to be affections of ordinary pon- 

 derable matter, the difficulty is only one of detail. 



If, again, we examine the, electricity of the atmosphere, 

 when, as is usually the case, it is positive with respect to that 

 of the earth, we find that each successive stratum is positive 

 to those below it and negative to those above it ; and the con- 

 verse is the case when the electricity of the atmosphere is 

 negative with respect to that of the earth. 



If another electrical phenomenon be selected, another sort 

 of change will be found to have taken place. The electric 

 spark, the brush, and similar phenomena, the old theories 

 regarded as actual emanations of the matter or fluid, Elec- 

 tricity ; I venture to regard them as produced by an emission 

 of the material itself from whence they issue, and a molecular 



