XX

Elements or Pharmacy.

part I.

fixed in its proportions. In decomposition this is extremelyuseful; for the insoluble product, being immediately sepa-rated, cannot oppose the further action of the decomposingsubstance, which would be the case were it to remain in so-lution.

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. Efflorescence may also influence chemical affinity; afact which was first observed by Scheele, who ascertained,that if in a paste composed of several saline substances decom-position is going on, one of the resulting compounds oftenrises through the mass, and forms an efflorescence on its sur-face ; and its being thus withdrawn from the sphere of actioncontributes towards forwarding the decomposition.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 ata higher temperature, owing to one or more of the componentshaving its affinity weakened by an increased elasticity. Thus,mercury exposed to air for some time at a temperatureequal to its boiling point, combines with the oxygen of the air,and is converted into red oxide of mercury; but if the fire beraised so as to make the retort red hot, this oxide is again de-composed, and running mercury and oxygen gas obtained.

From the influence of the above circumstances on chemicalcombination, the utility of these pharmaceutical and chemicaloperations, which diminish aggregation, overcome the effect