PART I.

Elements of Pharmacy.

xix

which they are soluble, than when their parts are mechanicallydivided; and this does not happen altogether from the merecircumstance of a larger surface being presented to the fluid; fornative oxide of tin, which in the aggregate resists completelythe action of any acid, becomes soluble when its aggregationis overcome by mechanical operations ; and some other sub-stances are similarly affected. Hence trituration, levigation,and granulation are ranked among pharmaceutical operations,and are of importance " in facilitating chemical action, partlyby diminishing aggregation, and partly by increasing the sur-face on which affinity is exerted." In some instances mechan-ical division is not sufficient, and recourse must be had to pre-cipitation. Thus liquid potash will not dissolve silica in powder,as it can be obtained by trituration; but when the silica isprecipitated from a state of chemical solution, it is readilydissolved in liquid potash.

Owing to the force of cohesion, also, solid bodies seldomact chemically on solids; while fluids readily combine withfluids, and likewise act with energy on solids for which theyhave an affinity. Fluidity, however, is not indispensable tochemical action; there being many cases in which two solids,in a state of minute mechanical division, act chemically oneach other. 1 (See Section iii.) When, however, the specificgravities of even two fluids are very materially different, theirchemical combination is opposed, to a certain extent, by theforce of cohesion of the heavier fluid ; and hence, agitation isfrequently necessary for aiding the operation of affinity.

Cohesion has sometimes a considerable influence in deter-mining the proportion of combinations formed in consequenceof new affinities. Thus, if its intensity be sufficient to counter-balance the affinity of the fluid in which the integrant particlesresulting from a new combination are formed, it will combinethese, and produce crystallizations or precipitations,-which,withdrawing the substance thus formed in part from thesphere of action, and opposing a resistance to any furtherexertion of chemical power, will consequently determine theproportions of the combination.

3. Insolubility must necesarily 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 thus

1 Thence the axiom Corpora nan ogunt nisi sint solula, which was foiinerN establiilwdin chemistry, is not gtneralh/ true.

a 2