6. Efflorescence may also influence chymical affinity-a fact which was first observed by Scheele, who ascertained, that if in a paste composed of several saline substances decomposition is going on, one of the resulting compounds often rises through the mass, and forms an efflorescence on its surface; and its being thus withdrawn from the sphere of action, contributes towards forwarding the decomposition.

7. The influence of temperature in modifying chymical action is very considerable. An increased temperature, by promoting fusion, and in other respects weakening the attraction of cohesion in solids, favours combination; but opposes it in some cases, inasmuch as it augments elasticity. In both instances its effects are much modified by the degree of its intensity; combinations effected at a lower being often dissolved at a higher temperature, owing to one or more of the components having its affinity weakened by an increased elasticity. Thus, mercury exposed to air, for some time, at a temperature equal to its boiling point, combines with the oxygen of the air, and is converted into red oxide of mercury; but, if the fire be raised so as to make the retort red-hot, this oxide is again decomposed, and running mercury and oxygen gas are obtained.

From the influence of the above circumstances on chymical combination, the utility of those pharmaceutical and chymical operations, which diminish aggregation, overcome the effect of specific gravity, diminish elasticity, and regulate temperature, such as pulverization, trituration, granulation, agitation, and compression, with the proper management of furnaces, is sufficiently obvious.

In that department of pharmacy, also, which regards extemporaneous composition, it is of importance to attend to the slowness with which chymical action is in many instances produced; for substances, which have mutual affinities for each other, may give no indication of any change when newly mixed, but yet, after some time, produce very complete changes. Such compounds, therefore, when they are intended to act medicinally, should be exhibited as soon as possible after they are made.

Chymical attraction may be exerted between more than two bodies, so as to bring three or four into one combination; and such compounds are named ternary, quaternary, etc. according to the number of their components. Several examples of these are to be found among the saline preparations (Part iii.); and almost all the vegetable substances are compounds of three or more principles.

The forces with which chymical attraction is exerted are different in different bodies. In cases where this attraction is exerted in a superior degree, by a third body, to that of either of the components of a compound of two bodies, so as to decompose it, and form a new compound, while, at the same time, one of the components of the previous compound is set free, the affinity thus exerted has been termed single elective attraction. To represent the relative forces of affinity, tables were first constructed by Geoffroy; and afterwards much improved and extended by other chymists, particularly Bergmann. The remarks of Berthollet may have tended to lessen their value in the opinions of some; but their utility to a certain extent must undoubtedly be acknowledged.1 When the elective attractions are more complicated, or when two elective affinities are exerted, and two new compounds formed, this is termed double elective attraction. In such cases, Mr. Kirwan denominated the attractions which tend to preserve a compound in its original state quiescent; while the others, which tend to separate the principles of a compound from each other, he termed divellent attractions.

As an example of double elective attraction, let it be supposed that two compounds in solution, one consisting of potassa and sulphuric acid, or sulphate of potassa, and the other consisting of chlorine and calcium, or chloride of calcium, be mixed together, a double decomposition will take place, and two new compounds, sulphate of lime, and chloride of potassium, will be formed. In this case, if the attraction between potassa and sulphuric acid be 62, and that between calcium and chlorine be 20, the sum of the quiescent attractions will be 82; but if the attraction between potassa and muriatic acid be 32, and that between sulphuric acid and lime be 54, the sum of the divellent attractions will be 86; which, exceeding the former sum of the quiescent, will operate and produce the above-stated decompositions and resulting compounds.

According to the opinions of Bergmann, the relative force of the affinities which produce these effects is capable of being measured, and the changes are altogether to be ascribed to the predominance of the affinities of one set of substances over those of another.

But the changes produced by the predominance of certain affinities over others, are ascribed by Berthollet to those circumstances which influence attraction, and limit combination. If four substances, for example, be presented to each other, two of which have a greater tendency to cohesion than the other two have, so as to form by their union an insoluble compound, instead of one compound being formed by the union of the four, in which the affinities are balanced, this will be averted by the force of cohesion, and the two which form the insoluble compound will unite, and be separated by precipitation or crystallization, leaving the other two in combination in the fluid which has been the medium of action. "If even these four substances were previously in the reverse binary combinations, on presenting them to each other, the affinities within the sphere of action must be reciprocally exerted; and the same extraneous forces will cause an exchange of principles, or the phenomena which have been ascribed to elective affinities will be produced." To avoid the term elective attraction, Berthollet denominates cases of this kind complex affinity.

The explanation of single elective attraction, or where three substances are presented to each other, is precisely the same; the union which takes place between two of them being determined by the tendency to cohesion, or the disposition of the combination of two of them to form a compound of little solubility.

1 See Appendix to Part I. No. IT.

Elasticity, likewise, has a considerable influence in determining decompositions where the application of heat is necessary; and according to Berthollet, the decomposition of a compound body of which one of the ingredients has a great tendency to assume the elastic form is to be ascribed to the disposition it has to escape from its combination, when aided by the intervention of even a weaker affinity.

In complex affinities the same cause determines the union of substances disposed to assume the elastic form, and separates them as a volatile compound. " If, therefore," says he, " it be desired to know the result of the exposure of two salts to the action of heat, it is only necessary to consider which of the two bases and which of the two acids have the greater volatility, if there be a difference; for the more volatile base and acid will escape and enter into combination, and the fixed base and fixed acid will remain behind, and combine with one another."1 Tables representing the forces of affinity have been constructed; but, as Dr. Henry has justly remarked, "one great obstacle to the construction of such tables is the difficulty of ascertaining with precision the quantities of bodies required for neutralization."2

A knowledge of the doctrines of affinity is of the utmost importance in pharmacy; and, as the foregoing sketch presents little more than an outline, 1 must refer those who would wish to investigate the subject to Thomson's and Murray's Systems of Chymistry, Bergmann's Dissertation on Elective Attraction, Berthollet's Researches into the Laws of Chymical Affinity, Richter's Foundation of Stochiometry, Sir Humphry Davy's Elements of Chymical Philosophy, and my friend Dr. Turner's Elements of Chymistry.

1 Researches, p. 3. quoted by Murray, System of Chymistry, i. 120.

2 Henry's Elements of Experimental Chymistry, 7th ed. vol i. p. 57.