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FIXITY being the opposite Qua∣lity to Volatility, what we have discoursed about the latter, will make the nature of the former more easily understood, and upon that account allow me to make somewhat the quicker dispatch of what I have to say of it.
The Qualifications that conduce most to the Fixity of a portion of mat∣ter, seem to be these.
First, the grossness or the bulk of the corpuscles it consists of. For if these be too big, they will be too unwieldly and unapt to be carried up into the Air by the action of such mi∣nute particles as those of the Fire, and will also be unfit to be buoyed up by the weight of the Air; as we see, that Vapours, whilst they are such, are small enough to swim in the Air, but can no longer be sustained by it, when they convene into drops of rain or flakes of snow. But here it is to be observed, that when I speak of the corpuscles that a fixt body consists of, I mean not either its Elementary or its Hypostatical Principles, as such, but onely those very little masses or clusters of particles, of what kind so∣ever they be, that stick so firmly to one another, as not to be divisible and dissipable by that degree of fire in which the body is said to be fixt; so that each of those little Concreti∣ons,
though it may it self be made up of two, three, or more particles of a simpler nature, is considered here per modum Ʋnius, or as one intire cor∣puscle. And this is one Qualification conducive to the Fixtness of a bo∣dy.
The next is the ponderousness or solidity of the corpuscles it is made up of. For if these be very solid, and (which solid and compact bodies usually are) of a considerable speci∣fick gravity, they will be too heavy to be carried up by the effluvia or the action of the fire, and their pon∣derousness will make them as unwiel∣dy, and indisposed to be elevated by such Agents, as the grossness of their bulk would make bigger corpuscles, but of a proportionably inferiour specifick weight. On which account the calces of some metals and mine∣rals, as Gold, Silver, &c. though, by the operation of Solvents, or of the fire, or of both, reduced to powders exceedingly subtile, will resist such vehement fires, as will easily drive
up bigger, but less heavy and com∣pact, corpuscles, than those calces consist of.
The third Qualification that con∣duces to the Fixity of a body, be∣longs to its Integral parts, not barely as they are several parts of it, but as they are aggregated or contexed in∣to one body. For, the Qualification, I mean, is the ineptitude of the com∣ponent corpuscles for avolation, by reason of their branchedness, irre∣gular figures, crookedness, or other inconvenient shape, which intangles the particles among one another, and makes them difficult to be extricated; by which means, if one of them do ascend, others, wherewith 'tis com∣plicated, must ascend with it; and, whatever be the account on which divers particles stick firmly together, the aggregate will be too heavy or unwieldy to be raised. Which I therefore take notice of, because that, though usually 'tis on the rough∣ness and irregularity of corpuscles, that their cohesion depends; yet it
sometimes happens, that the smooth∣ness and flatness of their surfaces makes them so stick together, as to resist a total divulsion; as may be illustrated by what I have said of the cohesion of polished marbles and the plates of glass, and by the fixity of glass it self in the fire.
From this account of the Causes or Requisites of Fixity, may be dedu∣ced the following means of giving or adding Fixation to a body, that was before either Volatile, or less fixt. These means may be reduced to two general Heads; First, the action of the Fire, as the parts of the body, exposed to it, are thereby made to operate variously on one another. And next, the association of the par∣ticles of a volatile body with those of some proper additament: Which term, [of proper] I rather imploy than that, one would expect, [of fixt;] because 'twill ere long appear, that, in certain cases, some volatile bodies may more conduce to the fixation of other volatile bodies, than some fixt
Ones doe. But these two Instruments of Fixation being but general, I shall propose four or five more particular ones.