COPPER, SILVER, AND GOLD 42 8/ 



CuS and As 2 S 2 in their ordinary insoluble forms and in the state of 

 the colloid solution of their hydrosols (see Chapter I., Note 57, and 

 Chapter XVII.,. Note 25 bis). Here, however, an important step in 

 advance has been made in this respect, that we are dealing with the 

 solution of a simple body, and moreover of a metal i.e. of a particu- 

 larly characteristic state of matter. And as boron, gold, and certain 

 other simple bodies have already been obtained in a soluble (colloid) 

 form, and as numerous organic compounds (albuminous substances, 

 gum, cellulose, starch, &c.) and inorganic substances are also known in 

 this form, it might be said that the colloid state (of hydrogels and hydro- 

 sols) can be acquired, if not by every substance, at all .events by sub<- 

 stances of most varied chemical character under particular conditions 

 of formation from solutions. And this being the case, we may hope 

 that a further study of soluble colloid compounds, which apparently 

 present various transitions towards emulsions, may throw a new light 

 upon the complex question of solutions, which forms one of the problems 

 of the present epoch of chemical science. Moreover, we may remark that 

 Spring (1890) clearly proved the colloid state of soluble silver by means 

 of dialysis as it did not pass through' the membrane. 



As regards the capacity of silver for chemical reactions, it is 

 remarkable for its small capacity for combination with oxygen, and for 

 its considerable energy of combination with sulphur, iodine, and cer- 

 tain kindred non-metals. Silver does not oxidise at any temperature, 

 and its oxide, Ag 2 0, is decomposed by heat. It is also a very impor- 

 tant fact that silver is not oxidised by oxygen either in the presence of 

 alkalis, even at exceedingly high temperatures, or in the presence of 

 acids at least, of dilute acids which properties render it a very 

 important metal in chemical industry for the fusion of alkalis, and also 

 for many purposes in everyday life ; for instance, for making spoons, 

 salt-cellars, <fec. Ozone, however, oxidises it. Of all acids nitric acid 

 has the greatest action on silver. The reaction is accompanied by the 

 formation of oxides of nitrogen and sjrver nitrate, AgNO 3 , which 

 dissolves in water and does not, therefore, hinder the further action of 

 the acid on the metal. The halogen acids, especially hydriodic acid, 

 act on silver, hydrogen being evolved ; but this action soon stops, 

 owing to the halogen compounds of silver being insoluble in water and 

 only very slightly soluble in acids ; they therefore preserve the remaining 

 mass of metal from the further action of the acid ; in consequence of 

 this the action of the halogen acids is only distinctly seen with finely- 

 divided silver. Sulphuric acid acts on silver in the same manner that 

 it does on copper, only it must be concentrated and at a higher 

 temperature. Sulphurous anhydride, and not hydrogen, is then evolved. 



