LITERARY AND SCIENTIFIC. 67 



this law there are some exceptions. Oxalic acid and lime unite in the dry state, 



and snow and salt will combine without being first melted. 



That chemical affinity is dependent on the electrical states of bodies, has been 

 rendered highly probable from the experiments of Sir H. Davy and others. 

 Substances that have ordinarily powerful affinities for each other may be separated 

 by the electric agency, and one body may be made to pass through another for 

 which it has commonly a strong affinity, without combining it— thus acids will 

 pass through alkaline solutions, and alkalis through acids, without combination. 



A knowledge of chemical affinities enables us to separate substances from each 

 other, and to unite others, so as to form new compounds. 



When one substance is employed to separate another from its combination, it is 

 effected by what is called single elective affinity^ as when sulphuric acid is added 

 to a solution of muriate of baryta — the baryta leaves the muriatic acid, combines 

 with the sulphuric acid, and is precipitated as the insoluble sulphate of baryta. 



When one compound, containing two substances, is employed to decompose 

 another compound, and two new compounds result from the decomposition, the 

 exchange is effected by what is termed double elective affinity : thus, when a solu- 

 tion of carbonate of potash, and a solution of sulphate of magnesia are mixed toge- 

 ther, the carbonic acid leaves the potash to combine with the magnesia, and form 

 the insoluble carbonate of magnesia which is precipitated — while the sulphuric 

 acid leaves the magnesia to combine with the potash, forming sulphate of potash. 



Bergman, who introduced these terms, conceived that chemical action was, in 

 all cases, owing to elective affinity, and, accoi-diugly, he drew up tables of affinity 

 between different bodies ; but Berthollet pointed ont the error of Bergman's 

 opinions, and shewed that chemical affinity is not invariably the same in all cases. 

 He attempted to prove, indeed, that it is always influenced by modifying circum- 

 stances, such as temperature, cohesion, elasticity, and quantity of matter. But in 

 this attempt Berthollet went rather too far. That chemical affinity is greatly 

 influenced by them is true — but still chemical affinity exists independent of them, 

 Thus when iron, lead, and silver are exposed to the action of oxygen or atmos- 

 pheric air, the iron readily unites with the oxygen, the lead gradually combines 

 with it, but the silver is scarcely affected by it ; and under the same circumstances 

 we find water much more disposed to combine with some substances than with 

 others ; with muriatic acid gas it combines much more freely than with carbonic 

 acid gas, and with the latter gas still more freely than with oxygen. Oil has no 

 disposition to combine with water, yet it readily unites with a solution of potash 

 forming soap. 



The effect of one of the modifying circumstances of Berthollet may be seen in 

 the following experiment : — On passing a stream of hydrogen gas over the oa^ide 

 of iron heated to redness, the oxygen of the iron will unite with the hydrogen, 

 and form water ; again, on passing the vapour of water over iron filings, heated to 

 redness, the iron will separate the oxygen from the vapour of water. From the 

 first experiment it follows that hydrogen has a stronger affinity for oxygen than 

 iron has ; but from the second, it would appear that iron has the strongest affinity 

 for oxygen, because it separates that element from its combination with hydrogen. 

 This alteration of affinities between hydrogen, oxygen, and iron, is probably 

 owing to an alteration in their electrical conditions effected by the agency of caloric. 



The effect of heat and elasticity in modifying chemical affinity, is exhibited 

 very clearly in the formation of carbonate of ammonia by sublimation, from 

 muriate of ammonia and carbonate of lime. If a solution of carbonate of ammo- 

 nia and muriate of lime be put together, a double decomposition will follow. The 

 carbonic acid will leave the ammonia to unite with the lime, forming an insoluble 

 precipitate of carbonate of lime, while the muriatic acid will leave the lime to 

 combine with the ammonia, forming muriate of ammonia. From what takes place 

 in this experiment it would scarcely be expected that carbonate of lime could be 

 made to decompose muriate of ammonia ; yet by putting these substances toge- 

 ther and applying heat, a decomposition is effected, and the muriatic acid again 

 combines with the lime, forming muriate of lime, while the carbonic acid leaves 

 the lime, and again unites with the ammonia, forming carbonate of ammonia. 



When bodies unite with each other, the resulting compounds always possess 

 properties differing from those of their constituents. Thus, highly corrosive and 

 pungent bodies often become tasteless and inodorous ; and, on the other hand, 

 tasteless and inodorous bodies become corrosive and pungent. Solids become 



