420 Mr. Hunt on the Spectral Images of M. Moser, 



having its origin in the sun, but broadly distinguished from it 

 by its producing no influence on the organs of sight, certainly 

 *' so modifies the surfaces of bodies that they condense vapours 

 otherwise than usual ;" and like modifications are produced by 

 condensing vapours on parts of the surface, or by placing 

 other bodies in contact with it. Whatever method we may 

 adopt to disturb the surface of any body, be it metallic or vi- 

 treous, we have an unequal condensation of vapour. If with- 

 out touching the surface of a metal plate, we subject it to the 

 disturbance produced by a blow or two on the back of the 

 plate, we shall find an irregular deposit of vapour if we breathe 

 on it. The molecular change which bodies undergo, under 

 the most trifling circumstances, is certainly one of the most 

 curious matters with which photography has brought us ac- 

 quainted. By light, by heat, by electricity we can dispose 

 plates to receive vapours over definite spaces; by lowering the 

 temperature of parts of any body, the same effect is produced, 

 and by any mechanical force we do the same. If we place a 

 copper plate so that one half of it shall rest on a cold body, 

 and apply the heat of a spirit lamp for a few seconds to the 

 other half, carefully avoiding touching the polished surface, 

 and then expose the plate, when quite cold, to mercurial va- 

 pour, it will be found that a larger quantity of vapour is depo- 

 sited over the half that was warmed than over the other half. 



If a piece of wood is placed upon a polished plate, and one 

 or two gentle blows is given to it, the plate will exhibit, when 

 submitted to vapour, not merely the shape of the wood, but a 

 perfect picture of its fibres. Again, if we give a metal plate a 

 few gentle blows upon the back, the surface will distinctly 

 show, when exposed to vapour, the spaces corresponding 

 with those on the back on which the hammer fell. If we sub- 

 ject portions of a metal plate to any chemical action, even 

 though it may be inappreciable to the sight, it will exhibit the 

 spaces to which the action was confined, the moment it is ex- 

 posed to the influence of vapour. 



These experiments afford us a sufficient amount of evidence 

 to conclude, that any cause producing a change upon solid sur- 

 faces disposes them to condense vapours unequally. They also 

 prove the correctness of all the statements made by M. Fizeau, 

 Professor Grove, Mr. Prater * and others ; and at the same 

 time as they do this, they convincingly show us, that these 

 observers have only been dealing with a few curious facts, 

 which cannot be allowed to explain these remarkable phaeno- 

 mena, to which, in particular, M. Moser wishes to direct at- 

 tention, viz. the power which the solar rays have of produ- 

 * See p. 225 of the present volume. — Edit. 



