THE PHOTOGRAPHIC IMAGE. 383 



active, aud it is possible to modify the silver haloids themselves so as 

 to make them seusitive for the red eud of the spectrum. 



The chemical nature of the colored products of photo-chemical decom- 

 positiou is still enshrouded in mystery. Beyond the fact that they con- 

 tain less halogen than the normal salt, we are not much in advance of 

 the knowledge bequeathed to us by Scheele in the last century. The 

 problem has been attacked by chemists again aud again, but its solu- 

 tion presents extraordinary difficulties. These products are never 

 formed — even under the most favorable conditions of division and with 

 prolonged periods of exposure — in quantities beyond what the chemist 

 would call "a mere trace." Their existence appears to be determined 

 by the great excess of unaltered haloid with which they are combined. 

 Were I to give free rein to the imagination I might set up the hypothesis 

 that the element silver is really a compound body invariably containing 

 a minute percentage of some other element which resembles the com- 

 pound which we now call silver in all its chemical reactions, but alone 

 is sensitive to light. I offer this suggestion for the consideration of the 

 si)eculative chemist.* For the colored product as a whole, *. e., the 

 product of photo-decomposition with its combined unchanged haloid, 

 Carey Lea has proposed the convenient term " photo-salt." It will avoid 

 circumlocution if we adopt this name. The photo-salts have been 

 thought at various times to contain metallic silver, allotropic silver, a 

 sub-haloid, such as argentous chloride, etc., or an oxy-haloid. The free- 

 metal theory is disposed of by the fact that silver chloride darkens 

 under nitric acid of sufficient strength to dissolve the metal freely. The 

 acid certainly retards the formation of the j)hoto-salt, but does not pre- 

 vent it altogether. When once formed the photo-chloride is but slowly 

 attacked by boiling dilute nitric acid, and from the dry photo-salt 

 mercury extracts no silver. The assumption of the existence of an 

 allotropic form of silver insoluble in nitric acid can not be seriously 

 maintained. The sub-haloid theory of the product may be true, but it 

 has not yet been established with that precision which the chemist has 

 a right to demand. We must have analyses giving not only the per- 

 centage of halogen, but also the percentage of silver, in order that it 

 may be ascertained whether the photo-salt contains anything besides 

 metal aud halogen. The same may be said of the oxy-haloid theory; 

 it may be true, but it has not been demonstrated. 



The oxy-haloid theory was first suggested by Robert Huntt for the 



* I have gone so far as to test this idea experhuentally in a i)relimiuary way, the 

 result being, as might have been anticipated, negative. Silver chloride, well dark- 

 ened by long exposure, was extracted with a hot saturated solution of potassium 

 chloride, and the dissolved portion, after precipitation by wat<'r, compared with the 

 ordinary chloride by exposure to light. Not the slightest difference was observable 

 either in the rate of coloration or in the colors of the products. P(U"haps it may be 

 thought worth while to repeat the experiment, using a method analogous to the 

 " method of fractionation " of Crookes. 



t •' Kesearches on Light," "^d ed., 1«54, p. 60. 



