Molecular Actions of Crystalline Particles^ §c. 1 03 



steatite, which possess but in a very slight degree the power of 

 fixing mercurial vapours. It appears therefore that the 

 power which water has of wetting glass, causes it to have a 

 greater tendency to deposit than mercury, which does not wet 

 glass. The cause of the production of thermographic images 

 is evidently similar to that which causes the deposition of a 

 solid body from a solution. 



The fixation of the mercurial vapours in the Daguerreo- 

 type process, which has excited so much interest, and for 

 which so many theories have been advanced, is but another 

 example of the force which causes the deposition of solid and 

 gaseous particles from a liquid, and which produces so many 

 other effects. In this case the chemical rays of light act in 

 the same manner as mechanical action and caloric in causing 

 a certain molecular disturbance. By the discoveries of Moser, 

 it is shown that these rays possess the power of acting upon 

 almost any body, in such a manner as to render it capable of 

 fixing the particles of various vapours. Thus simple minerals, 

 glass, &c. may be made to fix the mercurial vapour. 



It appears however that silver, gold, copper, &c, which form 

 amalgams, or in other words, are capable of being wetted by 

 mercury, possess this property in a greater degree than any 

 other bodies which are incapable of being wetted by it; in 

 the same way as we have seen that glass has the greatest 

 power to fix the vapour of water. Admitting the truth of this 

 theory of the Daguerreotype process, we are naturally led to 

 inquire whether the same agent may not likewise cause the 

 fixation of particles in a state of solution or of vapour, in the 

 same manner as by simple mechanical action. After several 

 unsatisfactory attempts, I finally succeeded in clearly proving 

 this fact. The solution which shows the influence of light the 

 most evidently, is that of the neutral chloride of gold. A few 

 grains of this salt dissolved in an ounce of water, when ex- 

 posed to the light, deposits minute crystals of a metallic ap- 

 pearance on that side of the glass nearest the light. 



The action of light in causing the deposition of gaseous va- 

 pours may be shown by placing some iodine in a bottle closed 

 with a glass stopper. After being exposed to the sunshine for 

 several hours, minute black crystals will appear on the side 

 nearest the light, which will change their position according 

 to the side of the glass exposed. Another substance which 

 shows this action still better, is camphor, a piece of which, 

 merely covered with a glass shade, will give rise to a crystal- 

 line deposit, after an hour or two of exposure to light, and 

 which presents the same phsenomena as that of iodine. By a 

 prolonged exposure these crystals become very abundant, and 



