COPPER, SILVER, AND GOLD 429 



then obtained as a white flocculent precipitate, silver bromide forms a 

 yellowish precipitate, and silver iodide has a very distinct yellow 

 colour. These halogen compounds sometimes occur in nature ; they 

 are formed by a dry method by the action of halogen compounds on 

 silver compounds, especially under the influence of heat. Silver chlo- 

 ride easily fuses at 451 on cooling from a molten state ; it forms 

 a somewhat soft horn-like mass which can be cut with a knife 

 and is known as horn silver. It volatilises at a higher tempera- 

 ture. Its ammoniacal solution, on the evaporation of the ammonia, 

 deposits crystalline chloride of silver, in octahedra. Bromide and 

 iodide of silver also appear in forms of the regular system, so that in 

 this respect the halogen salts of silver resemble the halogen salts of the 

 alkali metals; 24 



forming very unstable ammoniacal compounds. When heated, these compounds (Vol. I. 

 p. 250, Note 8) evolve the ammonia, as they also do under the action of all acids. Silver 

 chloride enters into double decomposition with potassium cyanide, forming a soluble 

 double cyanide, which we shall presently describe ; it also forms a soluble double salt, 

 NaAgS 2 O 3 , with sodium thiosulphate. 



Silver chloride offers different modifications in the structure of its molecule, as is seen 

 in the variations in the consistency of the precipitate, and in the differences in the action 

 of light which partially decomposes AgCl (see Note 25), Stas and Carey Lea investigated 

 this subject, which has a particular importance in photography, because silver bromide 

 also gives photo-salts. There is still much to be discovered in this respect, since Abney 

 showed that perfectly dry AgCl placed in a vacuum in the dark is not in the least acted 

 upon when subsequently exposed to light. 



24 Silver bromide and iodide (which occur as the minerals bromite and iodite) 

 resemble the chloride in many respects, but the degree of affinity of silver for iodine is 

 greater than that for chlorine and bromine, although less heat is evolved (see Note 28 Bis), 

 Deville deduced this fact from a number of experiments. Thus silver chloride, when 

 treated with hydriodic acid, evolves hydrochloric acid, and forms silver iodide. Finely- 

 divided silver easily liberates* hydrogen when treated with hydriodic acid ; it produces 

 the same decomposition with hydrochloric acid, but in a considerably less degree and 

 only on the surface. 'The difference between silver chloride and iodide is especially 

 remarkable, sinco the formation of the former is attended with a greater contraction 

 than that of the latter. The volume of AgCl = 26 ; of chlorine 27, of silver 10, the sum 

 = 87, hence a contraction has ensued j and in the formation of silver iodide an expansion 

 takes place, for the volume of Ag is 10, of I 26, and of Agl 89 instead of 86 (density, 

 AgCl, 5'59 ; Agl, 5'67). The atoms of chlorine have united with the atoms of silver 

 without moving asunder, whilst the atoms of iodine must have moved apart in 

 combining with, the silver. It is otherwise with respect to the metal ; the distance 

 between its atoms in the metal = 2-2, in silver chloride' =8'0, and in silver iodide 

 = 8'5 ; hence its atoms have moved asunder considerably in both cases. It is also very 

 remarkable, as Fizeau observed, that the density of silver iodide increases with a rise oi 

 temperature that is, a contraction takes place when it is heated and an expansion wheq 

 it is cooled. 



In order to explain the fact that in silver compounds the iodide is more stable than 

 the chloride and oxide, Professor N. N. Beketoff, in his '.Researches on the Phenomena 

 of Substitutions ' (Kharkoff, 1865), proposed the following original hypothesis, which we 

 will give in almost the words of the author: In the case of aluminium, the oxide, Al 9 0g, 

 is more stable than the chloride, A1 2 C1 6 , and the iodide, A1 2 I . In the oxide the amount 

 of the metal is to the amount of the element, combined with it as 54*8 (Al 87'8) iff to 48 



