374 Royal Institution. 



diminution of the colour greater than the mere dilution would have 

 produced, as was exemplified in the cases of the red sulphocyanide 

 of iron mixed with sulphate of potash, and of the scarlet bromide 

 of gold mixed with chloride of potassium. The lecturer accord- 

 ingly drew the conclusion that when two salts mix without preci- 

 pitation or volatilization, the acids and bases frequently, if not uni- 

 versally, arrange themselves according to some definite proportion ; 

 and that this depends on the relative quantity of the two salts, as 

 well as upon the proper affinities of the substances composing them. 

 He was unable then to enter upon the influence of heat, or of dilu- 

 tion in certain cases, or to add any remarks connected with double 

 salts, or with other metals, or upon certain practical applications of 

 these views in chemical and physiological science. 



The fact that we very frequently find the double decomposition of 

 a salt to be complete, the whole of one of its constituents being pre- 

 cipitated, was shown to be easily explained on the principles of 

 Berthollet. Thus, for instance, when chromate of potash and 

 nitrate of silver are mixed, at the first moment a division will take 

 place producing four salts, but one of these — the chromate of 

 silver — is thrown down at once as a precipitate, and thus put out of 

 the field of action. Another division of the acids with the bases 

 must take place, producing of course more of the insoluble chromate, 

 and so on, till at length the whole of the silver is removed. And 

 that this is really what does take place is rendered almost certain by 

 the fact that wherever by an interchange of acids and bases a pre- 

 cipitate can be produced, that precipitate does form; and, if the 

 substance be perfectly insoluble, the whole is thrown down ; this 

 occurring in opposition to all rules of " affinity," and to all tables 

 that Bergmann, or any other chemist, ever did or could construct. 

 The volatility of one of the products acts in the same manner as in- 

 solubility, as is exemplified in the decomposition of carbonates by 

 any other acid. Crystallization also is but another phase of the same 

 phagnomenon. An experiment was exhibited in illustration of this. 

 Dilute solutions of nitrate of Hme, and sulphate of soda, were mixed 

 at the ordinary temperature without producing any separation of 

 solid matter; but they were so proportioned that upon heating the 

 mixture, the crystallization of some sulphate of lime was determined, 

 and when once this had commenced, it progressed rapidly ; re- 

 sembling in that respect the ordinary phsenomena of precipitation. 

 If in a double decomposition a far larger quantity of a sparingly 

 soluble salt be produced at the first moment than the water can dis- 

 solve, the crystals will be formed rapidly and will accordingly be 

 very small in size ; but should there be formed at once only just 

 sufficient to determine a separation in the solid form, the crystals 

 will grow gradually, and will often attain a large size. Tliis was 

 exemplified on the mixture of nitrate of silver with the sulphates of 

 cojiper and of potash resnectively. 



It is possible that the law of Berthollet may not be universally 

 applicable ; yet the present advanced state of science shows that not 

 only is there, as Bergmann insisted, a true chemical affinity, that is. 



