40 Note on the Experiments of Moser. 



and which is very easy to repeat. A medal is taken, an en- 

 graved plate or the print of a seal, and it is tarnished with the 

 breath ; then, whilst this surface is <hus covered with a layer 

 of condensed vapour, one of Daguerre's plates is placed upon 

 it and left there for some instants ; it is then removed, and we 

 find the print of the medal, of the engraving, or the seal traced 

 upon the plate by the vapour which is unequally condensed 

 there. 



If the object subjected to the experiment had been covered 

 with a layer of mercury or of iodine, instead of being tarnished 

 with the breath, the same result would have been obtained. 

 In all cases in which the drawing is produced the vapour is 

 not deposited on the portions which are in intaglio, but this 

 difference disappears if the two surfaces are left a long time in 

 contact. It seems natural to admit that the nearest points are 

 those on which the condensation of the vapour first operates. 

 When a gold leaf is suspended in a vessel, at the bottom of 

 which there is some mercury, the leaf begins to grow white 

 on its under side which is nearest to the surface of the mer- 

 cury. When once an image is formed on a surface by the 

 unequal condensation of the vapour, it will be understood, ac- 

 cording to the principles already explained, how it is rendered 

 visible by the condensation of fresh vapour. 



If the medal, &c. is put in contact with the plate after being 

 heated, instead of covering it with vapour, the same phaeno- 

 menon takes place, that is to say, the images reappear. In 

 this case it may also be admitted, that by heating the medal, 

 &c. the veil of vapour which covered it has been forced to 

 disappear; it is the same phaenomenon which must take place 

 when a plate of metal or of glass is exposed to the direct 

 action of the solar rays, being covered with a perforated dia- 

 phragm which has emitted some vapour, the condensation of 

 which afterwards took place on the points of this diaphragm 

 in contact with the plate. That being allowed, the results 

 which have been obtained necessarily come under the cases 

 quoted. This explanation may be rendered more complete 

 by means of an experiment easy to execute, and which was 

 made in our laboratory. When we operate in vacuo, and let 

 the medals and other objects used in the experiment cool 

 there, the images by contact, which should become manifest 

 with the successive condensation of the vapours, are not pro- 

 duced. 



If we also admit (what seems in accordance with our know- 

 ledge on this point) that all the surfaces of bodies exposed to 

 the atmosphere are covered with a veil of moisture, — that this 

 veil must continually increase or diminish in passing from one 



