XV1U

Elements of Pharmacy.

part I.

density, to which, and the change of form suffered by oneor both of the components, its production may be ascrib-ed. Thus steam condensed to water parts with a largeportion of caloric; and the same effect is produced when wateris solidified by being mixed with quicklime. The contrary effect,however, or an absorption of caloi'ic, is also produced bychemical combination, when the density of the compound isless than the mean; as, for instance, when, by solutions of saltsin water, or in some other fluids, very intense colds, greaterthan any natural cold, are artificially produced. *

The exertion of chemical affinity is influenced by variouscircumstances: these, according to Berthollet, are mass, co-hesion, insolubility, specific gravity, elasticity, and inflores-cence.

1. That mass has a considerable share in influencing che-mical affinity was first suggested by Berthollet, who states it asa canon, that combinations do not depend altogether on theattraction of affinity, but on the proportions also, of the sub-stances brought into action. Thus, if a and b form a com-pound, and c be a substance which has a stronger affinity fora than b has, it should be able when mixed with the compoundto withdraw a altogether from b, if combination was regulatedby affinity only: but this he affirms is not the case in fact; forc does not entirely combine with a, but is shared between itand b, according to the force of the affinity, and the bulk ofeach. This view of the subject affords a reason why, in phar-maceutical compositions, a small quantity of a substance maybe added to a compound, without producing any sensible effect,although, if added in large quantity, decomposition would di-rectly ensue. It follows, also, from it, 1st. that " the chemicalaction of one substance on another, must diminish as it ad-vances to saturation :" and, 2dly, that a decomposing substance" must oppose a stronger resistance to the decomposing agent,in proportion as the decomposition proceeds, from the increasein the relative quantity of one of its ingredients to the other,which is abstracted:" and, lastly, " that in estimating the rela-tive forces of affinity in bodies, the quantities of them must betaken into account, and ought to be equal." Objections havebeen advanced to the opinions of Berthollet on this subject, yet,as the opposing theories are not fully established, it is unneces-sary to enter into an examination of the objections at the pre-sent moment.

2. Cohesion has an evident influence in opposing chemicalaction, and counteracting the exertion of chemical affinity.Thus all aggregates are more slowly acted on by liquids in

1 Sec Appendix to Part I. No. I.