1858.] Molecular Impressions hy Light and Electricity. 461 



an attenuated atmosphere of the vapours of phosphorus, this element 

 is changed by the electric discharge into its allotropic variety, which 

 is deposited in notable quantity on the sides of the receiver. In 

 this experiment, the transverse bands or striae discovered by Mr. 

 Grove, in 1852, are very strikingly shown. Not only is the gaseous 

 intermedium thus affected, but the terminals from which the dis- 

 charge appears to issue, are disintegrated, and their molecules 

 projected. Some tubes, through the interior of which Mr. Gassiot 

 had passed the discharge from RuhmkorfTs coil for a considerable 

 time, were shown to be coated in the interior, for a notable space 

 around the negative terminal, with a deposit of platinum, forming 

 a reflecting surface like the back of a looking-glass. The vacuum 

 in these tubes was Torricellian, the tubes having been hermetically 

 sealed after the descent of the mercury, so as to cut them off from 

 the mercurial surface. In these cases the electric discharge passes 

 from metal to metal ; but the glow which is seen on excited elec- 

 trics, such as glass, was also shown by Mr. Grove to be accompanied 

 with molecular change. Letters cut in paper, and placed between 

 two well cleaned sheets of glass, formed into a Ley den apparatus 

 by sheets of tin foil on their outer surfaces, and then electrified by 

 connexion for a few seconds with a Ruhmkorff coil, had invisible 

 images of the letters impressed upon the interior surfaces, which 

 were rendered visible by breathing on them ; and rendered visible, 

 and at the same time permanently etched, by exposure, after elec- 

 trization, to the vapour of hydrofluoric acid. 



So, again, if iodized collodion be poured over the surface of 

 glass having the invisible image, and then treated as for a photo- 

 graph, and exposed to uniform daylight, the invisible image is 

 ultimately developed in the collodion film ; the invisible mole- 

 cular change having been conveyed to the collodion, and rendering 

 it, when nitrated, more sensitive to light in the parts where it has 

 been in proximity to the electrical impression, than in the residual 

 parts. Here we have a molecular change, produced first by elec- 

 tricity on the glass, then communicated by the glass to the collodion, 

 then changed in character by light, and all this time invisible; 

 and then rendered visible by pyrogallic acid, the developing 

 chemical agent. Test papers between the plates of glass so elec- 

 trized, show an acid, and also a bleaching re-action, probably due 

 to the formation of nitrous acid and of ozone ; and thus evidencing 

 a chemical change in the elastic intermedium, as well as in the 

 bounding surfaces : but the interior molecules of the glass appear 

 also to partake of the effect, as the impressions are reproduced in 

 many cases on the opposite surface of the glass. 



Mr. Babbage had observed that some plates of glass which had 

 formed the ornamented margin of an old looking-glass, and were 

 backed by a design in gold leaf covered with plaster of Paris, 

 showed, when this backing was removed by soft soap, an impression 

 of the gold-leaf device, which was rendered visible by the breath on 



