If we compare the different bodies act ed on by gravitation, we shall find that the absolute force of their gravitation is in all cases the same, provided their dis tances from each other, and their mass, be the same ; but this is by no means the case with electrical and magnetic bodies : in them, the forces by which they are at tracted towards each other are exceed ingly various, even when the mass and the distance are the same. Sometimes these forces disappear almost entirely ; at other times they are exceedingly in tense.
Gravity, therefore, is a force inherent in bodies ; electricity and magnetism not so ; a circumstance which renders the opinion of their depending on peculiar fluids extremely probable. If we com pare the absolute force of these three powers with each other, it would appear that the intensity of the two last, every thing else being equal, is greater than that of the first; but their relative inten sity cannot be compared, and is therefore unknown. Hence it follows that these different attractions, though they follow the same laws of variation, are not the same in kind.
The attractions between bodies at in. sensible distances have been distinguish ed by the name of affinity, while the term attraction has been more commonly con fined to cases of sensible distance.
Affinity may be considered as operat ing on homogeneous or heterogeneous substances. Homogeneous affinity urges substances of the same nature together, as iron to iron, soda to soda. Hetero geneous affinity draws substances of dif ferent natures into union, as acid and al kalies.
Homogeneous affinity is usually deno minated cohesion, and sometimes adhe sion, when the surfaces of bodies are on ly referred to; it is nearly universal ; as far as is known, caloric and light alone are destitute of it.
Heterogeneous affinity is the cause of the formation of compound substances ; thus muriatic acid unites with soda, and forms sea-salt; and sea-salt in saturated solution is united into masses by homoge neous affinity. Heterogeneous affinity is universal as far as is known ; that is to say, there is no substance which is not attracted by some other substance. It is generally taken for granted, that every substance has more or less affinity for all others, though it is certainly assuming more than even analogy can warrant, and is a point which we have no means of as certaining.
Affinity, like sensible attraction, varies with the mass and the distance of the at tracting bodies. That cohesion varies with the mass cannot indeed be ascertained, because we have no means of varyingthe mass, without, at the same time, altering the distance. But in cases of the adhesion of the surfaces of homogeneous bodies, which is undoubtedly an instance of ho mogeneousaffinity, ithas been demonstra ted, that the force of adhesion increases with the surface, which, in some respect, is the same as with the mass.
That heterogeneous affinity increases with the mass has been observed long ago, in particular instances, and has been lately demonstrated by Berthollet to take place in every case. Thus, a given por tion of water is retained more obstinately by a large quantity of sulphuric acid, than by a small quantity. Oxygen is more ea sily abstracted from oxides which are ox ydised to a maximum, than from those which are oxyded to a minimum. Lime only takes off the greatest part of the car bonic acid from potash, which still retains a portion of it ; and sulphuric acid does not totally displace phosphoric acid from the lime united to it in phosphate of lime ; a part of it remains undisturbed. In these, and many other cases, a small por tion of one substance is retained by a given quantity of another more strongly than a large portion ; and Berthollet has shewn, that in all cases a large quantity of one substance is capable of abstracting a por tion of another from a small portion of a third, how weak soever the affinity be tween the first and second is, and how strong soever that between the second and third.
That the force of affinity increases as the distance diminishes, and the contrary, is obvious ; for it becomes insensible, whenever the distance is sensible, and, on the other hand, it becomes exceedingly great, when the distance is exceedingly diminished. But the particular rate which this variation follows is still unknown ; some have supposed the rate to be the same as that of sensible attraction, and that its intensity varies inversely, as the square of the distance ; no sufficent ar gument has ever been advanced, to prove this law to be incompatible with the phe nomena of affinity ; but, on the other hand, no proof has ever appeared in sup port of this opinion.