112 Action of certain Metals, $-c., on a Photographic Plate. 



Samples of commercial specimens of the following essential oils 

 have been tried : Peppermint, lemons, pine, juniper, bergamot, 

 winter green, lavender, cloves, eucalyptus, cajeput, and cedrat, 

 and were all found to be active, and not only when used by 

 themselves, but also when dissolved in a large amount of pure 

 alcohol. It is well known that the active substances in the essential 

 oils are bodies known as terpenes, and I have to thank Dr. Tilden for 

 supplying me with pure specimens of these bodies ; they are all of 

 them remarkably active. Paraldehyde and benzaldehyde are also 

 very active bodies. Ordinary aldehyde and formaldehyde, as they 

 have been at present applied, are only slightly active. Guaiacum, 

 both when in powder and in alcoholic solution, is active, and so are 

 powdered cinnamon, sweet spirits of nitre, and eau de cologne. 

 Brandy is slightly so. Now the important property belonging to all 

 th,ese bodies is their reducing or oxygen-absorbing power, hence the 

 conclusion that it is this property which enables them to act on the 

 photographic plate. The mineral oils, purified petroleum spirit, 

 aloohol, ether, the esters, such as ethyl acetate, benzene, nitrobenzene, 

 are all, when pure, unable to produce any effect on a sensitive plate, 

 and even oxidised bodies nearly related to the terpenes, such as ter- 

 pinol and camphor, are not active, neither is thymol. Terebene is 

 an exceedingly active body. The difference of activity of the ordi- 

 nary oils seems to follow their oxygen-absorbing power, at all events it 

 is so with regard to linseed and olive oils, for the former is the most 

 active of the oils, and 1 gram of it is said to be capable of absorbing 

 186 c.c. of oxygen, while the same weight of olive oil can absorb 

 only 8*2 c.c. It is also interesting to note that, at least with some of 

 these active bodies, results can be obtained which correspond to 

 what photographers term solarisation or reversal, the action, when 

 modified, giving a black picture, when carried to its full extent a 

 white one. 



It has already been stated that a mere trace of zinc makes mer- 

 cury active, but it is certainly equally curious and unexpected that 

 a trace of zinc should make alcohol active. It is only necessary to take 

 pure alcohol and place in it some strips of bright zinc foil, leaving 

 the metal in for three or four days. It will then be found that the 

 alcohol can act strongly on a photographic plate. The same happens 

 with ether and with ethylacetate, but not with benzene. In addition 

 to zinc, cadmium, magnesium, aluminium, and fusible metal can act 

 in the same way, whereas lead, nickel, tin, silver, sodium, and, as far 

 as experiments have gone, all the inactive metals, have no such 

 power. This reaction is still being investigated, but certain it is 

 that careful filtration does not remove this activity from alcohol, nor 

 does distillation entirely destroy it. 



