1. That quantity has a considerable share in influencing chymical affinity was first suggested by Berthollet, who states it as a canon, that combinations do not depend altogether on the attraction of affinity, but on the proportions also of the substances brought into action. Thus, if a and b form a compound, and c be a substance which has a stronger affinity for a than b has, it should be able, when mixed with a compound, to withdraw a altogether from b, if combination was regulated by affinity only: but this, he affirms, is not the case in fact; for c does not entirely combine with a, but is shared between it and b, according to the force of the affinity, and the bulk of each. This view of the subject affords a reason why, in pharmaceutical compositions, a small quantity of a substance may be added to a compound, without producing any sensible effect, although, if added in large quantity, decomposition would directly ensue : thence it follows, 1st, that "the chymical action of one substance on another must diminish as it advances 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 increase in the relative quantity of one of its ingredients to the other, which is abstracted;" and lastly, "that, in estimating the relative forces of affinity in bodies, the quantities of them must be taken into account, and ought to be equal." Objections of considerable weight have been advanced to the opinions of Berthollet on this subject, by Pfaffi

1 Sec Appendix to Part I. No. I.

Sir H. Davy, and others; but it is unnecessary to enter into an examination of these at the present moment; and we may only observe, that the theory of Berthollet, however plausible, is not unobjectionable, and also, that it is not in accordance with the results of experiment, as demonstrated by M. Dulong.

2. Cohesion has an evident influence in opposing chymical action, and counteracting the exertion of chymical affinity. Thus, all aggregates are more slowly acted on by liquids, in which they are soluble, than when their parts are mechanically divided: and this does not happen altogether from the mere circumstance of a larger surface being presented to the fluid; for native oxide of tin, which, in the aggregate, resists completely the action of any acid, becomes soluble when its aggregation is overcome by mechanical operations : and some other substances are similarly affected.

On account of the influence of mechanical division, we find trituration, levigation, and granulation ranked among pharmaceutical operations. They are of importance "in facilitating chymical action, partly by diminishing aggregation, and partly by increasing the surfaces on which affinity is exerted." In some instances mechanical division is not sufficient, and recourse must be had to precipitation. Thus, liquid potassa will not dissolve silica in powder obtained by trituration; but when the silica is precipitated from a state of chymical solution, it is readily dissolved in liquid potassa.

The force of cohesion may be lessened in two ways: namely, by the power of caloric, and by mechanical division. The first acts by producing liquefaction : for, owing to the force of cohesion, also, solid bodies seldom act chymically on solids; while fluids readily combine with fluids, and likewise act with energy on solids for which they have an affinity. Fluidity, however, is not indispensable to chymical action: there being many cases in which two solids, in a state of minute mechanical division, act chymically on one another.1 (See Section iii.)

When the specific gravities of two fluids are very materially different, their chymical combination is opposed, to a certain extent, by the force of cohesion of the heavier fluid; thence agitation is frequently necessary for aiding the operation of affinity.

Cohesion has sometimes a considerable influence in determining the proportion of combination formed in consequence of new affinities. Thus, if its intensity be sufficient to counterbalance the affinity of the fluid in which the integrant particles resulting from a new combination are formed, it will combine these, and produce crystallizations or precipitations, which, withdrawing the substance thus formed in part from the sphere of action, and opposing a resistance to any further exertion of chymical power, will, consequently, determine the proportions of the combination.

1 Thence the axiom Corpora turn agunt nisi sint solula, which was formerly e Aahlished in chymistry, is not generally true.

S. Insolubility must necessarily modify chymical action. If an insoluble compound substance be acted on by any substance tending to combine with one of its principles, this is protected, in some degree, by the insolubility of the compound withdrawing it from the action of the decomposing substance; and if a compound, which is produced in the progress of combination, be insoluble, it will be directly precipitated, and thus fixed in its proportions. This is illustrated when a solution of a sulphate of zinc is decomposed by acetate of lead: the sulphuric acid and the oxide of lead are both withdrawn, by their combination forming an insoluble compound, namely, sulphate of lead, whilst acetate of zinc is obtained in solution. In decomposition this is extremely useful; for the insoluble product, being immediately separated, cannot oppose the further action of the decomposing substance, which would be the case were it to remain in solution.

4. Specific gravity influences considerably the exertion of affinity, particularly if the substance be of little solubility, by withdrawing it from the sphere of action, and hence retarding its combinations; and in many instances this can be but imperfectly counteracted by agitation.

5. Chymical attraction, as far as the aeriform substances are concerned, is opposed by elasticity. Thus, when two gases having mutual affinities are mixed together, they very seldom combine, which is ascribed to the distances between the particles of substances existing in the gaseous state; for, as chymical attraction is exerted at insensible distances only, the particles of the two gases, although mingled together, are yet without the sphere of attraction. That this is owing to elasticity, is evident from the circumstance that the vapours which are not elastic more readily combine. Hence, whatever gives density to highly elastic substances, as, for example, mechanical pressure, or cold to a certain degree, must favour their chymical combination.