Elements of chemistry : including the most recent discoveries and applications of the science to medicine and pharmacy, and to the arts / by Robert Kane.

  • Robert John Kane
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry : including the most recent discoveries and applications of the science to medicine and pharmacy, and to the arts / by Robert Kane. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

    32/716
    tallized are not soluble, or it may be wished to obtain crystals oth- erwise than by solution. By meltino- a solid substance, its particles are allowed liberty of motion; and when it again commences to solidify, they may arrange themselves regularly, and crystallize. Almost all bodies, when melted, and then allowed to solidiiy, do thus crystallize; but the spaces left between the crystals which first form being completely filled up by the portions which solidify afterward, there remains only a general crystalline structure, visi- ble in the fracture of the body. Thus cast iron, sulphur, zinc, &c., have crystalline fractures. The beautiful feathered appearance given to sheet tin by washing with dilute acid, and which was so popular some years ago under the name of moiree metallique^ was simply this crystalline structure, displayed by removing the thin layer of metal on the outside, which had solidified too rapidly to have acquired any trace oi crystallization. To obtain, therefore, the metals crystallized by fusion, the excess of liquid metal must be removed from around the crystals that are first formed. A quantity of the ductal or of sulphur, having been melted in a cup, is to be allowed to cool until a solid crust has formed upon the surface and at the sides to a certain depth; two apertures must then be made in the upper crust, and the fluid metal T^^Tv^r-^/^PT^ remaining be poured out at the one aperture, while '^ ' v-'/ H ^j^jg jjjj. enters at the other to supply its place. On then breaking the vessel, the interior of the solid feil^iv-^^—''-l ^^y^^ ^^ metal or sulphur is generally found lined is>^^// with well-formed and characteristic crystals, as rep- > —^ resented in the figure. Bodies may also be crystallized by sublimation. When a sub- stance has been converted into vapour, and that, in condensing, it assumes at once the solid form, its particles arrange themselves so as to form crystals. Thus arc obtained in fine crystals, arsenic, arsenious acid, corrosive sublimate, benzoic acid, &c. It frequently happens that the same body may be obtained crys- tallized by more than one of these processes. Thus, corrosive sub- limate may be crystallized by solution or by sublimation ; sulphur may be crystallized either by fusion or by solution. It is remark- able that, Avhen this occurs, the crystals obtained by the two pro- cesses are never of the same shape ; they have not even any simple relation of figure to one another, but indicate a totally different mode of arrangement of particles, induced probably, at least in part, by the different temperatures at which the change of state of aggregation may have occurred. A body which cry^stallizes thus in two ways is said to be dimorphous, and this character will be found hereafter of the highest importance in the theory of the atomic constitution of compound bodies. The more slowly the change of state occurs, the more reo-i,]ar and the larger, are the crystals that are formed. Hence, in practice' solutions are left to cool very slowly, or to evaporate spontaneously ' atid sublimation is eflected by the most gentle heat that can be ad- vantageously applied. To favour the deposition of the particles a variety of artificial acids may be applied. Thus, crystallization takes place better in a pan with some little roughness at the sides