If a solid be united with and suspend ed in a fluid, the former is said to be dissolved, or in solution, and the fluid is called a solvent. In this manner water dissolves sugar or salt. Fluids in gene ral dissolves greater quantities the higher the temperature, probably from the fluid state being promoted by heat.
Some substances unite in all propor tions, such as most acids in water ; but others have a limit ; as, for example, water will dissolve only one-fourth of its weight of common salt. It is then said to be saturated. But chemists use the word saturation in another sense. When two principles, as, for instance, an acid and an alkali, are combined, the properties of each disappear when a due proportion of each is nresent : but if either of the principles exceed that proportion, tile predominating property will be that of the principle which is in excess In these cases, the principles are said to be satu rated when the properties are most com. pletely balanced ; but in the other cases; the principle which is defective in quan. tity is said to be super-saturated, or over saturated, and the other principle which is in excess, is said to be under-saturated: acids united with alkalies manifest these cases very strikingly.
In the consideration of the phenomena of chemical or elective attraction between the principles of bodies, it will be very difficult to select instances for illustra tion, which shall be simple, either as to the principles or the effects, because in almost every case there is a degree of com plexity which obtains in nature ; and even where we suppose a great simplicity of principles, they may hereafter be dis covered to be compound. But the doc trine will be understood, and fixed in the memory, by the examples to be brought; in the same manner as when diagrams are used by geometricians, though the actual figures cannot strictly agree with their hypotheses or postu lates.
1. The mutual action of two bodies ex hibits the phenomena of simple elective attraction and rejection : when two prin ciples are presented to each other, they may either combine or reject each other. Thus water combines with and dissolves gum, but rejects camphor ; and alcohol combines with and dissolves camphor, but rejects gum.
It is probable, that all simple bodies, if insulated, would combine together, and that the phenomenon of rejection, when it takes place, is an effect of some of the compound elective attractions,uponwhicb we shall presently speak.
When a body is suspended to satura tion in a solvent, no more can be taken up or supported, because the cohesive attraction, or that of the parts of the body to each other, is stronger than that of the fluid to the same ; and it is found that the power of the solvent is gieater the less it is charged, until it ceases at the point of saturation. Elas ticity, or the energies by which bodies are converted into gas or vapour, is like wise an opponent to solution or combina tion, and gives a point of saturation which may be varied, by preventing or impeding the assumption of the elastic state.
2. When three bodies or principles are presented to each other in succession, we may conceive a variety of results, all which appear to take place in nature. Thus (a) they may not perceptibly unite, or (b) two may unite, and the third have no action,either upon them singly or when in their combination ; or (c) all three may unite, from attractions exerted between each singly upon the others, and form a triple compound ; or (d) two of them may have no attraction for each other, but be ing both capable of combining with a third, this last may be the instrument of union between the two, and a triple corn.
pound will be thus formed. In this case, the effect is said to be performed by in termediate attraction, and the attracting body is called a medium. Thus oil and water will not unite, but either of them will unite with an alkali ; and if this last be united Ni ith oil, it forms soap, which can be united with water. The alkali is the medinm ; or (e) two principles which attract each other may neither of them be capable of direct or ready union with a third ; but when the two former shall be actually combined together,the compound shall attract and combine with the third body, and form a triple compound. This new power is called resulting attraction. Thus neither sulphur nor potash have any sensible action upon gold ; but when they are fused together, they combine with that metal. ?lost of the effects of resulting attraction are consequences of the change of state of bodies, particularly to that of fluidity ; the effects of this attraction and that by a medium often ex ist in the same case ; or if we sup pose three principles to be in such cir cumstances of aggregation or tempera ture, as to have no perceptible disposition to unite in pairs, but that the resulting at traction of a compound of two of them, if united, would then act upon the third, and produce a triple combination, it may happen that this resulting attraction, which seems to be only in prospect, shall have power to complete the triple com pound; and the modification is called disposing attraction. Thus vinegar has no perceptible action upon copper, but it can dissolve the compound of copper and oxygen, called the oxide of copper : nei ther vinegar nor copper have any dispo sition to take oxygen from its elastic state in the atmosphere, so that copper and vi negar may be kept together without solu tion in a closed vessel : but if the air be admitted, the presence of the vinegar will dispose the copper to take oxygen and form an oxide, and with this combination the vinegar will unite. There is mucii convenience in the term, disposing attrac tion, as used to express this phenomenon, though it must be confessed that this prospective disposition, ascribed to un conscious beings, seems to produce sorhe confusion in the mind. It may therefore be proper to notice, that the case seems to belong to disposing attraction, and may be thus hypothetically explained. Copper, and several other metals, which attract oxygen from the air, become covered with a thin oxide or rust, which prevents any farther access of that fluid,and consequent ly it rusts no farther, unless the thin coat of oxide be scraped off and a new surface exposed; and if this were continued to be clone, all the copper would be gradually oxided. Now the vinegar, by the condition of our case, does this, and the copper is gradually and totally dissolved ; not, as it appears, because the copper and oxygen are disposed to unite by a third power, which, as it were, waits for them, but because this power removes an impedi ment, which would impede their progres sive union.