Experimental notes of the mechanical origine or production of fixtness.

CHAP. II.

THE next qualification requisite in the corpuscles of Volatile bodies is, that they be not too solid or heavy. For if they be so, though their bulk be very small, yet, unless other Circumstances do much com∣pensate their weight, 'twill be very difficult to elevate them, because of the great disproportion of their spe∣cific gravity to that of the Air, (which contributes to sustain and e∣ven raise many sorts of volatile parts) and to the strength of the igneous effluvia or other agents that would carry them up. Thus we see, that filings of Lead or Iron, and even Minium (which is the calx of Lead) though the grains they consist of be very small, will not easily be blown up like common dust, or meal, or o∣ther powders made of less ponderous materials.

A third Qualification to be desired in the corpuscles that should make

up a Volatile body is, that they be conveniently shaped for motion. For if they be of branched, hook'd, or other very irregular or inconvenient figures, they will be apt to be stopt and detained by other bodies, or en∣tangled among themselves, and con∣sequently very difficult to be carried upwards, in regard that, whilst they are thus fastened either to one ano∣ther, or to any stable body, each single Corpuscle is not onely to be considered, as having its own pecu∣liar bulk, since its cohesion with the other corpuscle or body that detains it, makes them fit to be look'd upon per modum Ʋnius; that degree of heat they are exposed to being pre∣sumed uncapable of disjoyning them. And this may be one Reason, why Water, though it be specifically hea∣vier than Oil, yet is much more easily brought to exhale in the form of va∣pours than is Oil, whose corpuscles by the lasting stains they leave on cloath, wood, wool, &c. (which wa∣ter will but transiently moisten, not

stain) seems to be of very intangling figures.

The fourth and last qualification requisite in a Volatile body is, that the parts do loosely adhere, or at least be united in such a manner, as does not much indispose them to be separated by the fire in the form of fumes or vapours.

For he that considers the matter, will easily grant, that, if the contex∣ture of the corpuscles, whereof a bo∣dy consists, be intricate, or their co∣hesion strong, their mutual implica∣tion, or their adherence to each o∣ther, will make one part hinder ano∣ther from flying separately away, and their conjunction will make them too heavy or unweildy to be elevated to∣gether, as intire though compounded parts. Thus we see, that in Spring, or the beginning of Summer, a wind, though not faint, is unable to carry off the lightest leaves of trees, be∣cause they stick fast to the bows and twigs on which they grow, but in Autumn, when that adhesion ceases,

and the leaves sit but loosely on, a wind no stronger than that they re∣sisted before, will with ease blow them off, and perhaps carry them up a good way into the Air. But here note, that it was not without some cause, that I added above, that in a fluid body, the parts should at least be united in such a manner, as does not much indispose them to be sepa∣rated. For 'tis not impossible, that the parts of a body may, by the fi∣gures and smoothness of the surfaces, be sufficiently apt to be put into mo∣tion, and yet be indisposed to admit such a motion as would totally sepa∣rate them and make them fly up into the Air. As, if you take two pieces of very flat and well-polished marble or glass, and lay them one upon the other, you easily make them slide a∣long each others surfaces, but not easily pull up one of them, whilest the other continues its station. And when Glass is in the state of fusion, the parts of it will easily slide along each other, (as is usual in those of o∣ther

fluids) and consequently change places, and yet the continuity of the whole is not intirely broken, but every corpuscle does somewhere touch some other corpuscle, and thereby maintain the cohesion that indisposes it for that intire separation accompanied with a motion upwards that we call avolation. And so, when Salt-peter alone, is in a Cruci∣ble exposed to the fire, though a ve∣ry moderate degree of it will suffice to bring the Salt to a state of fusion, and consequently to put the corpu∣scles that compose it into a restless motion; yet a greater degree of heat, than is necessary to melt it, will not extricate so much as the Spirits, and make them fly away.