(g) The case of attraction, which has most particularly engaged the attention of chemistry, is that, where two principles being combined are separated from each other by the addition of a third, which combines with one of them. This has, perhaps improperly, been called simple elective attraction, and by others precipi tating attraction : its principal effects or distinguishing character would, it seems, be better designated by the term de composing attraction. Thus, if sulphuric acid and magnesia be combined in the salt called sulphate of magnesia (dissolved in water) and potash be added, the acid will unite with this last, and the magnesia will be separated and fall down. It was for a long time thought that these combi nations and separations were complete and entire ; but they appear in every in stance to form cases of the intermedi ate or resulting attractions, wherein the proportions of the soluble and insoluble parts are extremely different, and the de grees of saturation often modify the re sults. For the body separated has al ways a small proportion of the two others, and the new compound usually suspended is not binary, but triple at least ; and the proportions and effects are more or less altered by the quantity of solvent present, and the aptitude of the new combinations to take the solid, fluid, or elastic states.
Tables of separation or decomposi 4on have been called tables of simple elec tive attraction. They are usually drawn up to express effects in the humid way with moderate proportions of water at the mid dle atmospheric temperature ; and in the dry way by the operation of fire acting upon the containing vessels, to raise their temperature. From the reasons just mentioned, they cannot be admitted as denoting invariable effects, but they are nevertheless useful, provided the modifi cations of circumstances be attended to in our general reasoning. See ELECTIVE ATTRACTION.
3, In our present discussion it was not practicable, from the nature of the sub ject to avoid presenting facts for illus tration, in which more than thitee princi ples were concerned; though the doc trines to be elucidated supposed no more than that number to be present. This supposition can admit only acme combi natio! , either of two or .of three princi ples; but in the complex effects of che mical attraction, four or more bodies may be treated of as forming distinct and se parate combinations ; and these COM pounds being presented to each other, may be affected by all the habitudes and circumstances upon which we have so long dwelled, besides such others as arise from their greater complexity. These cannot be here Tully treated. It will be sufficient at present to overlook those effects wherein compounds of many principles may be formed, or in which the intermediate, or resulting, or dis posing attractions may operate, and re gard only the cases in which two binary compounds, being presented to each other, do either remain unaltered, or else exchange their principles so as to form two other binary compounds. A few years ago this was thought to compre hend the greatest part of the doctrine of chemical attractions ; but, as practical science advances, the supposed simplici ty of the facts becomes less than before.
These phenomena, afforded by two bi nary compounds, have been classed un der the denomination of effects of dou ble elective attraction. These facts may be considered with regard to the whole force of the attractions that tend to re tain the original compounds, which have been called quiescent attractions, and the whole force of the attractions that tend to produce two new binary compounds, which have been called divellent attrac tions. If the former be the greatest, the change will not take place ; but if the }atter exceed, it will. Thus, if to the sulphate of potash lime he presented, the sulphuric acid being more strongly at tracted by potash than by lime, no de composition will ensue; but if muriate of lime be presented to the sulphate of potash, the lime will not only attract the sulphuric acid, but the muriatie, acid will attract the potash ; and the sum of the divellent attractions, namely, of the lime to the sulphuric acid, of the muri atic acid to the potash, being greater than the sum of the original or quiescent attractions; namely, of the sulphuric acid to the potash, and of the muriatic acid to the lime ; two new compounds, namely, sulphate of lime and muriate of potash, will be formed. See ELECTIVE ATTRACTION.
The most essential difference between the complicated cases of attraction here described, and those treated of just be fore, is, that the principles in these last are either saturated, or nearly so, when presented to each other ; and from this difference, and the number of principles, it is, that the effect of solvents, the force of cohesion and of elasticity, as well as of temperature, and other circumstances, act with more effect than in the simpler cases.
Whenever the cohesive attraction ope rates so as to form solid aggregates, whether by the congelation of fused bo dies by cooling, or the deposition of bo dies from their solvents, the aggregates, if not disturbed by too rapid condensa tion, or by other causes, have the form of solids bounded by flat surfaces, meet ing each other in certain definite angles. These solids are called crystals. The property of crystallizing seems to be a natural consequence of the resulting at tractions. For if a binary compound be attracted by any other principle or com pound, and the time and circumstances allow the particles to turn round, it ap pears obvious that the appulse and ad hesion will be made by such sides of the bodies as'are occupied by particles most strongly attractive of each other ; and this regularity of opposition must pro duce regularity of figure. See CRYSTAL LIZATION.
After this general statement of the means and agents of chemistry, it re mains only for us, in this general article, to give an outline of the different sub stances or principles upon which the pro cesses of nature and art are performed, and upon which the articles devoted to each may be consulted.