SECT, xxv.] EXPLANATIONS OF VARIOUS PHENOMENA. 271 



taken place to a considerable depth. Mr. Hunt's experi- 

 ments prove that the electro-negative metals make the most 

 decided images upon electro-negative plates, and vice versd. 

 M. Matteucci has shown that a discharge of electricity does 

 not visibly affect a polished silver plate, but that it pro- 

 duces an alteration which renders it capable of condensing 

 vapour. 



M. Fizeau ascribes a numerous class of these phenomena 

 to the action of a slight layer of organic or fatty matter on 

 the surfaces, which, being volatile, is transferred to any body 

 near, in a greater or less quantity according to the distance; 

 that is, according as the surface projects or sinks into hol- 

 lows. When the different parts of a surface are unequally 

 soiled by extraneous bodies, even in the minutest quantity, 

 the condensation of mercurial vapour is effected in a manner 

 visibly different on its different parts, and therefore images 

 are formed. Although this explains various phenomena, it 

 does not apply to those already described, as Mr. Hunt had 

 taken the precaution to divest the substances he used of 

 every trace of organic matter. 



It is difficult to see to what cause Mr. Hunt's experi- 

 ments on the reciprocal action of bodies in total darkness 

 can be attributed, unless, perhaps, to a constant radiation 

 of some peculiar principle from their surfaces, which really 

 seems to exist. 



The impression of an engraving was made by laying it 

 face downwards on a silver plate iodized, and placing an 

 amalgamated copper plate upon it : it was left in darkness 

 fifteen hours, when an impression of the engraving had been 

 made on the amalgamated plate, through the paper. 



As the same may be obtained on plates of iron, zinc, 

 or lead, it is evident that this result is not the effect of 

 chemical rays. 



An iodized silver plate was placed in darkness with a coil 

 of string laid on it, and with a polished silver plate suspended 

 one-eighth of an inch above it ; after four hours they were 



