224 ANNUAL OF SCIENTIFIC DISCOVERY. 



of one of its constituents being precipitated, was shown to be easily 

 explained on the principles of Berthollet. Thus, for instance, when chro- 

 raate 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 chro- 

 mate 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 precipitate 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 " affin- 

 ity," 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 insolubility, as is exemplified in the decomposition of carbonates 

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

 phenomenon. An experiment w r as exhibited in illustration of this. 

 Dilute solutions of nitrate of lime 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 crystal- 

 lization of some sulphate of lime was determined ; and when once this 

 had commenced, it progressed rapidly, resembling in that respect the 

 ordinary phenomena of precipitation. If in a double decomposition a far 

 larger quantity cf a sparingly soluble salt be produced at the first moment 

 than the water can dissolve, 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. This was exem- 

 plified on the mixture of nitrate of silver with the sulphates of copper and 

 of potash respectively. 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, a preference of one substance to combine with a certain other substance 

 instead of a third, but, in a great number of instances at least, this sub- 

 stance will combine with both according to certain proportions, whenever 

 the whole of the affinities can be brought into play at the same time. 



ON OSMOTIC FORCE. 



The following is an abstract of the Bakerian lecture on the above subject, 

 delivered before the lioyal Society by Prof. Graham : 



This name was applied to the, power by which liquids are impelled 

 through moist membrane and other porous septa in experiments of endos- 

 mose and exosmose. It was shown that, with a solution of salt on one side 

 of the porous septum, and pure water on the other side, (the condition of 

 the psmometer of Dutrochet when filled with a saline solution and im- 



