154 ATTRACTION. 



of soda, and sulphate of magnesia, is said to undergo double decom- 

 position, at the freezing temperature, producing sulphate of soda, and 

 muriate of magnesia.* It cannot be doubted, that electric and gal- 

 vanic agencies are frequently developed by heat, and that thus chem- 

 ical action is often induced. 



(v.) Jl modified degree of heat is necessary. Red precipitate is 

 formed at or near the boiling heat of mercury, but it is decomposed by 

 ignition, and both its oxygen and metal are recovered. 



(w.) Chemical action is often brought on by mechanical means. 

 Several of the fulminating powders, and the mixtures of the chlo- 

 rate of potash and combustibles, explode by a blow, by friction, and 

 pressure ; which favor, at once, the approximation of the particles 

 within the sphere of attraction, and the developement of heat which 

 favors the chemical action. 



(a?.) JVo approximation, short of imperceptible distance, will bring 

 on chemical action. The negative is established by the approxima- 

 tion of any kind of matter towards any other for which it has an 

 affinity ; as for instance, a drop of nitric acid on a glass plate, will be 

 indifferent to silver or copper filings pushed near to it, but the action 

 commences when apparent contact is established. When sulphur 

 and mercury are in apparent contact, there is no action, but it is 

 brought on by rubbing them together. Sulphuric acid will run to 

 the bottom of alcohol, and produce action only at the touching sur- 

 faces, but it is quickly brought on, in the entire mass, by agitation. 

 Agitation of fluids and solids, to make them mingle quickly, pro- 

 motes their action, as in the case of common salt and water. 



(y.) Even apparent contact is often insufficient, and solution be- 

 comes necessary. Hence, the old maxim, " corpora non agunt nisi 

 sint soluta." Tartaric acid and carbonate of soda, dry quicklime and 

 dry muriate of ammonia, dry nitrate of copper, wrapped in tinfoil 

 in each of these cases there is no action till moisture is supplied, 

 when it comes on vigorously. 



(z.) Bodies having no affinity are sometimes brought to unite by a third 

 body. Oil and water, by the intermedium of caustic alkali, form soap. 



(AA.) THE FORCE OF AFFINITY is DIFFERENT BETWEEN DIF- 

 FERENT BODIES. 



Were it otherwise, there w r ould be no decompositions, except by 

 the effect of the imponderable agents. 



(BB.) ELECTIVE AFFINITY is THE FIGURATIVE EXPRESSION OF 



THE PREFERENCE WHICH ONE BODY IS SUPPOSED TO MANIFEST 

 FOR ANOTHER, TO THE EXCLUSION OF A THIRD. 



The alcoholic solution of camphor is precipitated by water, which 

 unites with the alcohol, and the camphor may be redissolved by the 

 addition of more alcohol. 



* Aikiu's Diet. Vol. II. pp. 389, and 779. 



