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body, there are no two whofe force of affinityappears to be equal. This may in part dependon the refpefilive differences of their cohefiveattraction, but is no doubt principally owing toa real difference in the energy of their chemicalaffinity. Moll fubftances exift in vifible maffes,and therefore under the influence of coheflon ;but as chemical affinity takes place betweenheterogeneous elementary particles, it is obviousthat tliefe two attractions are always in oppo-fition to each other ; hence there is a priori apoffibility, which is alfo confirmed by experi-ment, that the particles of one body may havean affinity for thofe of another, and yet on ac-count of the fuperior coheflon of the two fub-ftances, the chemical attraction may be imper-ceptible. It is impoffible therefore to afcertainwhether every fpecies of particles has a chemicalaffinity for all the other fpecies, although it isfor the molt part the opinion of the belt cbemiftsthat this is really the cafe.

Heterogeneous affinity may be convenientlydivided into firnple, where only two Ample fub-ftances are concerned; ftngle elective, whereonly three are concerned ; and compound, wherea greater number is brought into aCtion.

Before, however, we proceed to enumeratethe charaCteriftic circumftances of each, it willbe neceffary to explain what is here meant by aJimple fubftance. The ancients believed manybodies to be Ample which the fuperior Ikill andknowledge of modern chemifts have molt af-furedly decompofed; and there is no reafon tobelieve that in any one cafe has chemical analyfisbeen able to procure the real elements or Ampleconftituent parts of fubftances. A chemicalelement, therefore, does not fo much fignify abody that is abfolutely undecompofable, as onethat has not yet been refolved. But there isanother fenfe in which even bodies that areknown to be compound may be confldered asAmple, and that is where they combine withother fubftances, without being themfelves de-compofed. Thus fulphuric acid, which conflftsof oxygen and fulphur, becaufe it unites withearths, alkalies, and metallic oxyds, withoutbeing decompofed, may in thefe cafes be con-fldered as a£ting the part of a Ample fubftance,and would no doubt have been fo efteemed, ifall our knowledge of this acid was conflned tothe phenomena which it exhibits when adfingon the above mentioned fubftances. In thisfenfe alone we ufe the term when fpeaking ofchemical affinity.

Simple affinity takes place when only twofpecies of particles, either really Ample or at

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leaft acting as fuch, are brought in contact witheach other. In this cafe there are obviouilytwo contending forces, that of coheflon tendingto retain each fpecies of particles in diftinfiftmaffes, and that of compofltion or chemicalaffinity tending to blend, and intimately uniteeach particle of one fpecies with one of theother. If both fubftances are folid, there willbe no combination, whatever may be the ftrengthof their mutual affinity, for the contact in thisinftance can only be fuperficial, and even thisvery imperfefifly. Thus iron and fulphur, whichin favourable circumftances will unite with great'energy, are wholly incapable of action if eachis in a folid form. Now the folidity or coheflonof a body may be reduced in two ways, eitherimperfectly, by mechanical means, fuch aspounding, cutting, flling, &c. Or more com-pletely by the appflcation of heat, it being theuniform effefih of this agent to enlarge the bulkand weaken the coheflon of all bodies that areexpofed to its action. It is from long experi-ence of the efleets of coheflon in preventingchemical union, that the axiom has originated,that “ bodies do not a£t on each other exceptthey are liquid;” which, although like manyof the old chemical maxims, it is not to be re-ceived without certain modifications, is yet, inmany cafes, Unfitly true : the efficacy of affinity-being ceteris paribus inverfely as the attractionof coheflon, and the fluidity of one at leaft ofthe fubftances, is in all cafes neceffary to che-mical compofltion. An example of Ampleaffinity is when quickfilver and melted fulphurare blended together. Here, the coheflon ofthe fulphur being weakened by fufion, and thequickfilver being alfo fluid, the quiefeent forceor attrafition of coheflon, is unequal to the reci-procal affinity between the fulphur and themetal, which in confequence becomes effica-cious, and combines the two into one body.All the cafes of folution in which no decompo-fion of the menftruum or the folvent takes place,are examples alfo of Ample affinity. Thus whencommon fait is thrown into water, the chemi-cal affinity of the two being ftronger than theirrefpefilive cohefive attractions, a compound isproduced in which the particles of fait waterare no longer diftinguifhable by the eye, norfeparable from each other by any mechanicalforce.

While a lump of fait is diffolving in water,it is obvious that the particles thus taken upwill be conftan.tly approaching towards anequality with thpfe of the water, fo that thernafs of fait, every fquare inch of whofe furface