CIILORJDE-BEKTHOLLErs LAWS 439 



As a proof that double decompositions like the above are actually 

 ;ir-o mpli>h'd in the sense of Berthollet's doctrine, the fact may be cited 

 that table salt may be entirely decomposed by nitric acid, and nitre may 

 be completely decomposed by hydrochloric acid, just as they are decom- 

 IIOMM! by sulphuric acid ; but this only takes place when, in the first 

 instance, an excess of nitric acid is taken, and in the second instance with 

 an excess of hydrochloric acid for a given quantity of the sodium salt, 

 and if the resultant acid passes off. If sodium chloride be put into a 

 porcelain evaporating basin, nitric acid be added to it, and the mix- 

 ture be heated, then both hydrochloric and nitric acids are expelled by 

 the heat. Thus the nitric acid partially acts on the sodium chloride, 

 but on heating, as both acids are volatile, they are both converted into 

 vapour ; and therefore the residue will contain a mixture of a certain 

 quantity 'of the sodium chloride taken and of the sodium nitrate formed. 

 If a fresh quantity of nitric acid be then added, reaction will again set 

 in, a certain portion of hydrochloric acid is again evolved, and on heat- 

 ing is expelled together with nitric acid. If this be repeated several 

 times, it is possible to expel all the hydrochloric acid, and to obtain 

 sodium nitrate only in the residue. If, on the contrary, we take 

 sodium nitrate and add hydrochloric acid to it in an aqueous solution, 

 then reaction again sets in, a certain quantity of the hydrochloric acid 

 displaces a portion of the nitric acid, and on heating the excess of 

 hydrochloric acid passes away with the nitric acid formed. On repeat- 

 ing this process, it is possible to displace the nitric acid with an excess 

 of hydrochloric acid, just as it was possible to displace the hydrochloric 

 acid by an excess of nitric acid. The influence of the mass of the sub- 

 stance in action and the influence of volatility is here very distinctly 



colour, while a solution of copper chloride is green. If we mix both salts together the 

 green tint is distinctly visible, so that by this means the presence of the copper chloride 

 in the solution of copper sulphate is clearly seen. If now we add a solution of common 

 salt to a solution of copper sulphate a green coloration is obtained, which indicates the 

 formation of copper chloride. In this instance it is not separated, but it is immediately 

 formed on the addition of common salt, as it should be according to Berthollet's doctrine. 

 The complete formation of a metallic chloride from common salt can only occur, 

 judging from the above, when it separates from the sphere of action. The salts of silver 

 are instances in question, because the silver chloride is insoluble in water; and therefore 

 if we add a solution of sodium chloride to a solution of a silver salt, then silver chloride 

 and the sodium salt of that acid which was in the silver salt are formed. The amount 

 of silver chloride formed immediately separates from the solution, because it is in- 

 soluble in water, and the reaction in consequence is further directed to the end that is, 

 either the whole quantity of the silver is separated or all the chlorine passes into silver 

 chloride. This method is made use of for separating silver from its solutions, and also 

 for determining the quantity of chlorine. It must not, however, be forgotten that silver 

 chloride is slightly soluble in water and still more so in a solution of sodium chloride, and 

 that, therefore, a certain trace of silver escapes precipitation. 



