Exp. In this case, the atoms of the outer surface do not immediately and closely unite, so as to prevent the internal caloric from flowing through it, but only so much as to give it a slow and regular passage; and hence, by the heat, the atoms of the body are made to assume a position more distant and uniform than that which they had before the operation, and the body is therefore made softer, and more flexible. The difference between this fact and the preceding, is readily accounted for, by admitting a slight difference in the extent and force of the constituent atoms of the body.

PH. 45. Some solids are hard and brittle.

Exp. This will happen either when the atoms of the surface are in firm contact, and union, while the interior parts contain ethereal atoms in a compressed state, as in ph. 43, or when the particles of the body throughout are united firmly in but few points, which may occur in a multitude of cases.

PH. 46. Some solids are soft and inelastic.

Exp. This must occur when the atoms, or particles composing the body, are but slightly united in many points; the cohesion being feeble, and easily yielding either way to a slender force applied.

PH. 47. Some solids are very elastic, as well-tempered steel, ivory, glass, &c.

Exp. This, the atoms of the body being of a suitable kind, will be the consequence of an arrangement intermediate between those noticed in the two foregoing cases; but because the elasticity of solids has never yet received a satisfactory explanation, I shall be a little more particular.

Let AB, EF, and CD, (fig. 41. plate II.) represent three rows of atoms in one plane, making part of a body, bent from a straight line to a curve. Now, by the act of

bending, the centers of the atoms on the concave side in AB are brought nearer, and therefore more into the spheres of each other's repulsion, while those on the convex side, CD, have their centers removed more distant in the sphere of each other's attraction. Hence, there is produced a force of attraction on the convex side, and of repulsion on the concave, both tending to bring the bent solid to its state of equilibrium, as in its former position and figure, and since there is an innumerable number of forces on both sides, and all tending to promote the same effect, the figure is restored in many cases with very great energy; for although in this instance, only three rows of atoms are represented, yet in nature there are an innumerable multitude of such rows. It will be observed, that the effect of the repulsion on the concave side will be much greater than that of the attraction on the other side, because as the centers separate on the convex side the attraction diminishes, but as they approach on the concave side, the repulsion immensely increases, because of the very small distance.

"The major part of the philosophers, who have attempted to give a theory of elasticity, have especially considered, that when an elastic body is bent, for example to an arc, the particles situated on the convex side, become further separated from one another, while those, which are on the concave side approach each other. But of all the causes on which the re-establishment of the body in its first state has been made to depend, such as attraction, the resistance of a particular subtile matter diffused between the moleculæ of bodies, the action of caloric, &c. there is not any which is conducive to a satisfactory explanation of the phenomena.”—Haüy.

Of this wonderful and hitherto puzzling phenomenon, the reader is now presented with another explanation, which, it is presumed will prove satisfactory to the phi

losopher, it being natural, and in unison with the circumstances of the action.

PH. 48. If a slender piece of steel be made red hot, and quenched in cold water, and if afterwards a gentle heat be applied till its temperature be sufficiently raised, it will be elastic, and form a spring.

Exp. By making the steel hot, and quenching it in water, it is rendered hard and brittle, as in ph. 43, the atoms of the surface are brought close together, and the body will break by the least fracture of the surface; but by gently heating it, the atoms composing the surface, are in a small degree removed farther apart, and some portion of the inclosed ethereal atoms are diffused among them; when this is carried to a certain point, a slight bending will not separate or break the surface, and it becomes fit for a spring by attaining the conditions of the last phenomenon. If the hardened steel be made too hot, the atoms are separated so much at the surface as greatly to diminish the elasticity, bringing the body into the state mentioned in ph. 44, and 46, the atoms being so far removed from each other, that the centers on the concave side are not brought by the bending sufficiently into the spherules of repulsion; and on the convex side there is great freedom of access for ethereal matter, and this lessens the attraction, which also is very much weakened on account of the greater distance.

: PH. 49. A spring, when in use, frequently breaks suddenly, and this occurs chiefly in very cold weather.

Exp. If from a quick movement of the spring, any part of the convex surface should have its contiguous atoms so far separated, that the ethereal matter can rush between them on every part, the spring ought to break according to the foregoing explanations, and this is most likely to happen in cold weather, when the surface is most contracted by the diminished temperature.

PH. 50. In raising the temperature of the steel through various degrees, it assumes successively the prismatic colours.

Exp. As the temperature is raised, the superficial atoms are more and more separated, and the inclosed ethereal atoms become more and more diffused as the heat increases; hence the surface acquires different conditions for the reflection of different sorts of ethereal atoms, and the different colours, doubtless, arises from the difference in the forces and spherules of the ethereal matter, which constitutes light, together with the greater or less velocity with which it is projected, and hence according to the state of the surface, we shall have all the different colours.

PH. 51. Ductility and malleability, or the capacity of bodies to be drawn out into wire, flattened, or extended, by pressing, rolling, hammering, &c. belongs chiefly to the metals in some, as gold, silver, &c. it is very great; while other metals, as tungsten, antimony, &c. are very brittle.

Exp. Bodies will be malleable and ductile, when their component atoms are capable of gliding over each other, without a separation by the accession of ethereal matter between them, and hence the parts will adhere together, while variously disposed, and distributed in the body. But when the body is such, that, on a slight displacement of its constituent atoms, the ethereal matter gains access between them, the body will easily break; on this account not only many of the metals, but several other bodies, are brittle every one is aware of this property in glass, separate its parts but in a small degree, and immediately the newly exposed surfaces are covered with ethereal matter, and the glass is cracked.

PH. 52. Malleability and transparency, are seldom found in the same body.

Exp. For malleable bodies (ph. 51.) may have their component atoms situated in every diversity of arrangement, producing an irregularity among the parts of the body which prevents the transmission of light these bodies also appear to contain a less quantum of ethereal matter in their pores, a circumstance which is also very unfavourable to transparency.

PH. 53. Glass vessels, if exposed to the cold air as soon as formed, generally break, but if well annealed, by being cooled very gradually, they generally remain firm.

Exp. When rapidly cooled, the parts unite first firmly at the surface, inclosing much ethereal matter, as in ph. 43, and as the cooling will be effected sooner, on some parts of the surface than on others, the ethereal matter will be pressed and forced by the uniting of these parts towards the opposite side, and on a slight alteration of temperature will produce a sudden fracture.-But when well annealed, the tenacious atoms of the body take a more uniform arrangement, and the ethereal ones are hence regularly dispersed, and have greater freedom, so that the vessel becomes permanently firm.

PH. 54. When melted glass in a liquid state, is dropped from a tube or ladle into cold water, it forms a solid piece of glass, having a head of considerable thickness, and is terminated on the opposite side by a slender part called the tail, it is sometimes denominated a glass tear, from its resemblance in some respects to a tear; the head may be struck with considerable force by repeated blows of a hammer without producing fracture.

Exp. The formation of the drop will be easily comprehended, since, as the parts which first enter the water are cooled, the ethereal matter ascends towards the opposite side, where it has greater freedom of exit, causing the glass to be drawn out in length that way: and the sur