10

Elements of Pharmacy.

part I.

sphere of action, and opposing a resistance to any furtherexertion of chemical power, will consequently determine theproportions of the combination.

3. Insolubility must necessarily modify chemical action. Ifan insoluble compound substance be acted on by any substancetending to combine with one of its principles, this is protectedin some degree by the insolubility of the compound with-drawing it from the action of the decomposing substance;and if a compound which is produced in the progress of com-bination be insoluble, it will be directly precipitated, and thusfixed in its pi'oportions. In decomposition this is extremelyuseful; for the insoluble product, being immediately sepa-rated, cannot oppose the further action of the decompos-ing substance, which would be the case were it to remain insolution.

4. Specific gravity influences considerably the exertion ofaffinity, particularly if the substance be of little solubility, bywithdrawing it from the sphere of action, and hence retardingits combinations; and in many instances this can be but im-perfectly counteracted by agitation.

5. Chemical attraction, as far as the aeriform substances areconcerned, is opposed by elasticity. Thus, when two gases,having mutual affinities, are mixed together, they very seldomcombine, which is ascribed to the distances between the par-ticles of substances existing in the gaseous state : for, as che-mical attraction is exerted at insensible distances only, theparticles of the two gases, although mingled together, are yetwithout the sphere of attraction. That this is owing to elas-ticity, is evident from the circumstance that the vapourswhich are not elastic more readily combine. Hence, what-ever gives density to highly elastic substances, as, for example,mechanical pressure, or cold to a certain degree, must favourtheir chemical combination.

6. Florescence may also influence chemical affinity; a factwhich was first observed by Scheele, who ascertained, that ifin a paste composed of several saline substances decompositionis going on, one of the resulting compounds often risesthrough the mass, and forms an efflorescence on its surface;and its being thus withdrawn from the sphere of action contri-butes towards forwarding the decomposition.

7. The influence of temperature in modifying chemical actionis very considerable. An increased temperature, by promot-ing fusion, and in other respects weakening the attraction ofcohesion in solids, favours combination; but opposes it insome cases, as much as it augments elasticity. In both in-stances its effects are much modified by the degree of its inten-sity ; combinations effected at a lower, being often dissolved at