that of the opaque acid does not, the former must contain the acid in the amorphous state. Green glass, kept for several hours or for days at a red heat, just trong enough to soften it, acquires a fibrous texture, the change proceeding gradually from the surface towards the interior, and is finally converted into the substance called Reaumur's porcelain, which is white, slightly transparent, mostly of fibrous fracture, specifically heavier, less fusible, and much harder than the glass out of which it has been formed. This change, which takes place without loss of weight, may be attributed to a crystalline arrangement of the atoms brought about by keeping the glass for a long time in a soft state. (Vid. Silicium.) Basalt, which is an aggregate of crystalline granules, is converted by fusion into a black glass; and this when kept for a long time at a red heat becomes once more fine-grained and opaque. A mixture of clay, lime, and magnetic iron ore, yielded when intensely heated a black slag; this, after it had solidified and nearly cooled, suddenly became red hot, and fell to pieces in the state of a fine grey powder. (Fuchs) Sugar which has been melted, as in barley-sugar and bonbons, is in a vitreous condition, but after some time becomes white and opaque. If melted sugar is allowed to cool to about 38° C. or 100 Fah., and then, while it is yet soft and viscid, rapidly and frequently extended and doubled up, till at last it consists of threads, its temperature rises in two minutes from 104° to 176° Fah., and it then consists of minute crystalline grains of a pearly lustre. (Graham, Elements of Chemistry, page 43.) Vesuvian and calcareous garnet (grossularia) have the same composition: for example, the Vesuvian, and garnet from the Wiluifluss have the same formula, 3Ca O, Al2 O3, 3Si O2, excepting that in both minerals part of the alumina is replaced by peroxide of iron which is isomorphous with it. Vesuvian, which crystallizes in square prisms, has a specific gravity of 34; garnet, which crystallizes in rhombic dodecahedrons, a specific gravity of 3.63. These dimorphous minerals both pass by fusion into the amorphous state; they both, when subjected to this treatment, yield without loss of weight exactly the same product, viz. a glass of the same green colour and the same degree of transparency as the crystallized minerals-but softer, and having a sp. gr. of only 2.95; so that in passing from the crystalline to the amorphous state garnet suffers an expansion of about one-fifth, and vesuvian of about one-seventh. The glass is also easily soluble in hydrochloric acid, which is not the case with either of the minerals. [Comp. Magnus, (Pogg. 20, 477; 21, 50; 22, 391); Hess (Pogg. 45, 341); Varrentrapp (Pogg. 45, 343.)] Axinite has a sp. gr. of 3.294, and is but very imperfectly decomposed by acids; after fusion, which takes place without loss of weight, it has a sp. gr. of 2.815, and is then easily dissolved by acids; this greater solubility it acquires even at a heat not sufficient to fuse it, but only to bake it together. (Rammelsberg, Pogg. 50, 363.) Many other crystalline siliceous minerals not soluble in acids become so by fusion, probably from the same cause. The following amorphous bodies, on the contrary, appear to be brought into the crystalline state when subjected to a heat not sufficient to melt them. In undergoing this change they exhibit, as first observed by Berzelius, a vivid incandescence, commencing at the point most strongly heated and extending through the entire mass; they are after wards found to possess greater specific gravity, greater hardness, and less solubility. The incandescence here observed is similar to the phenomena observed by Graham and Regnault (pp. 104 and 106). Most of the bodies which exhibit this phenomenon are originally crystalline, but when heated become porous and amorphous in consequence of losing water, ammonia, or other volatile matters: then, when no farther loss of weight takes place, if they are heated to a temperature a little below redness, they exhibit the incandescence just spoken of. To this class belong-the hydrates of zirconia, titanic acid, tantalic acid, molybdous oxide, oxide of chromium, peroxide of iron, and oxide of rhodium; also hydrated basic perarseniate of iron, hydrated antimonite of cobalt, hydrated antimoniate of cobalt, hydrated antimoniate of copper, and euxenite (which chiefly consists of hydrated tantalate of yttria.) If these compounds are heated merely to the point at which they part with all their water, they are afterwards nearly as soluble as in their hydrated state; but if by stronger heating incandescence has been produced, they are afterwards found to be much less soluble, and often alter in colour. Zirconia, after being heated to incandescence, is no longer soluble in any acid excepting boiling oil of vitriol; oxide of chromium after ignition is of a paler green than before, and soluble in nothing but boiling oil of vitriol; ignited peroxide of iron is as hard and as difficultly soluble as micaceous iron ore, which is crystallized peroxide of iron. The abovementioned antimonites and antimoniates are in their original state easily decomposed by hydrochloric acid, but after ignition they resist its action almost entirely, and are also of a much paler colour than before. Hydrated phosphate of magnesia and ammonia, after being deprived of all its water and ammonia by gentle heat, exhibits incandescence when more strongly heated. The same appearance is presented by the carburet of iron which remains in the distillatory apparatus after gently heating prussian blue or protocyanide of iron. Gadolinite (silicate of yttria) whose conchoidal fracture and obsidianlike appearance testify sufficiently to its amorphous structure, (although some persons have fancied that they could discover a crystalline structure in it,) exhibits vivid incandescence when moderately heated; it sustains thereby no loss of weight, but is afterwards found to dissolve but very imperfectly in hydrochloric acid even after several days' boiling, although before ignition it is very easily soluble. According to Kobell (J. pr. Chem. 1, 91), its specific gravity increases by this change only from 4.25 to 4:31. Th. Scherer also (Pogg. 51, 493) observed in the gadolinite of Ytterby but a very trifling increase of specific gravity, but he attributes this circumstance to the partially distingrated and impure condition of the mineral; for (a) gadolinite from Hitterön, (b) orthite from Fille-Fjeld, and (c) allanite from Jotun-Fjeld (which two minerals have nearly the same composition as godolinite) showed after ignition considerable increase of density; the sp. gr. of a increased from 4:35 to 4.63, giving a condensation of 1000 to 9395; that of b from 3.65 to 3.94, therefore condensation =9264; and of c from 3:54 to 3.76, therefore condensation 9417. The quantity of water lost by a was 0.18 per cent., that lost by b and c was greater. At the same time a lost by ignition its black colour and opacity, becoming bottlegreen and translucent. Scherer endeavours to explain the increase of density by supposing that the spherical atoms are at first disposed one upon another in such a manner that each spherule rests upon two others below it, and that the atoms after being heated are so displaced that each one of them rests upon three below it; this would give a condensation of from 1000 to 9432. Perhaps also the fact of gypsum dehydrated by gentle heat becoming hard when mixed with water, and the non-production of this effect if the gypsum has been strongly burnt, may be explained by supposing that this substance when deprived of its water by a moderate heat is in the amorphous, but after strong ignition in the crystalline state. b. Differences in the properties of compounds, probably arising from the different grouping of the Simple Atoms which make up a compound atom. In considering the different properties of compound bodies resulting from dimorphism and amorphism, the structure of the compound atoms has been supposed to continue unaltered, and the occurrence of this or that crystalline form, or of the amorphous state, to depend solely on the manner in which these compound atoms are arranged amongst themselves. Hence it follows that these dimorphous and amorphous conditions may occur in simple bodies also, since simple atoms as well as compound ones may be disposed amongst themselves in various ways, moreover that these several conditions (with perhaps some exceptions requiring further investigation) may be destroyed by fusion, evaporation, or solution of the solid body in which they exist; and it will then depend upon circumstances in what particular state the body will resume its solid form.— But it is otherwise with the differences now to be considered, the cause of which we shall assume to be that the manner or number in which simple atoms combine to form a compound atom may differ in different substances. It is easy to see that the various conditions hereby produced can occur only in compound bodies, and may remain unaltered by the passage of a body from the solid to the fluid state or vice versa; for the compound atoms once constructed in this or that particular way may, without disturbance of internal structure, form fluid compounds with heat or with ponderable solvents. With this difference of grouping of simple atoms in the formation of compound atoms are connected many striking differences not only in the physical properties but also in the chemical relations of the bodies concerned. a. Isomerism. When two or more compounds which exhibit different physical and chemical relations, are so constituted that their compound atoms must contain the same clements combined according to the same numbers of simple atoms, and there is no ground for supposing that their proximate elements are of different natures, these compounds are said to be isomeric (using the word in its narrowest sense). It is supposed that the simple atoms which form a compound atom are put together in different ways. Many of the compounds formerly classed under this head are now regarded as polymeric; it is only with regard to phosphoric acid, tellurous acid and telluric acid, peroxide of tin, and tartaric acid, that isomeric conditions are at present recognized; and even these bodies may with more or less probability be regarded as polymeric. Phosphoric acid, P O3, exhibits the three isomeric states of ordinary, pyro-and meta-phosphoric acid. Among the many diversities exhibited by these three acids, the most remarkable is that they saturate different quantities of a salifiable base. PO5 in the state of ordinary phosphoric acid saturates 3 atoms of a base, in the state of pyrophosphoric is situates 2 atoms, and in the state of metaphosphoric acid only 1 atom. If 1 atom of P O3, in whichever of the three states it may happen to be, is ignited with three atoms of any base, soda for instance, an ordinary phosphate is produced; but when P O is ignited with 2 atoms of soda, the result is a pyrophosphate, and with 1 atom a metaphosphate. By long digestion or boiling with a large quantity of water, which itself acts as a base, the pyrophosphates and metaphosphates are converted into ordinary phosphates. The particular quantity of base with which P O is in contact, seems then at certain high temperatures to affect the manner in which 1 atom of phosphorus arranges itself with respect to 5 atoms of oxygen, so that the compound is capable of saturating sometimes 3 atoms of a base, sometimes 2, and sometimes only one. If we would explain these remarkable relations of phosphoric acid first discovered by Graham on the hypothesis of Polymerism, we might consider ordinary phosphoric acid as P O', pyrophosphoric acid as P2 O10, and metaphosphoric acid as P2 015; then P O would saturate three, P2 O10 four, and P3 O15 three atoms of a base. (Vid. Phosphorus.) In the case of tellurous acid, Te O2, a more soluble modification A and a less soluble modification B are to be distinguished: the latter is produced by the action of nitric acid upon A, and is again converted into A by fusion with caustic potash. Telluric acid, Te O3, exhibits two modifications perfectly similar to the above; the more soluble of the two is converted into the less soluble, when two or more atoms of it are fused with one atom of potash; this is analogous to the transformations of phosphoric acid above noticed. If these compounds are regarded as polymeric, they must be supposed to exist as Te O2, Te2 O1, Te O3 and Te3 0°. (Vid. Tellurium.) Antimonious and antimonic acid may possibly pass from one such modification to the other when their salts are ignited. Peroxide of tin, when precipitated by caustic alkalis from a solution of the bichloride, is much more easily soluble in acids than the anomalous variety of it produced by the action of nitric acid upon metallic tin; the latter when dissolved exhibits also very different relations. Possibly the soluble oxide may be Sn O2 and the anomalous variety Sn2 O'. Among organic compounds the following may be regarded as isomeric: Tartaric and racemic acid (C2 H2 O');-mucic and paramucic acid (C H1 O1);—maleic and paramaleic acid (C1 H O3). B. Polymerism. Two or more compounds possessing different physical and chemical properties, and composed of the same elements in the same proportions are said to be polymeric, when their differences may be explained by supposing that their compound atoms contain different numbers of simple atoms, varying however in such a manner that the numerical ratio of the several kinds of simple atoms remains unaltered. If for example one of a group of polymeric compounds contains 1 atom of a substance A and 3 atoms of a substance B, then the second may contain 2 atoms of A and 6 atoms of B, the third 3 atoms of A and 9 atoms of B and so on. In such cases the weight of the compound atom varies, but the proportions between its elements remains the same. Besides the instances mentioned under Isomerism, which ought all perhaps to be included under this head, the following among organic combinations must be particularly noticed. Polymeric compounds all containing 1 part of hydrogen united with 6 parts of carbon, and therefore containing C H: Olefiant gas, the more volatile oil of oil-gas, rock-oil, eupion, oil distilled from bees'-wax, caout choucine, heveene, oil of wine, stearoptin of oil of roses, paraffin, cetine, &c. Olefiant gas is probably C H', cetine C H32. Benzin, oilgas-camphor, and scheererite are C2 H. The oils of turpentine, juniper, copaiba, lemons, and black pepper are C3 H1. Naphthaline and paranaphthaline are C3 H2. Cyanic acid is C N O, fulminic acid probably C N2 O2, cyanuric acid C N O3. Volatile chloride of cyanogen is C2 N Cl, the fixed chloride probably C6 N3 C13. 7. Metamerism. This term is applied by Berzelius to the case in which the compound atoms of two chemical compounds containing the same elementary atoms, and for the most part in the same proportions, are nevertheless made up of different proximate elements. According to this definition metameric bodies must always belong to the higher orders of compounds. In some cases one only of the bodies concerned is a compound of a higher order. It is only among organic bodies that metameric compounds occur. The following are the most important: Acetic acid Water CHO 2 1 3 сно 6 6 4 Formic ether Acetate of methylic ether 664 Formic ether and acetate of methylic ether have the same specific gravity when in the form of vapour, and nearly the same specific gravity and boiling point when in the liquid state: but in other respects they are totally different; the former, when treated with caustic potash, is resolved into formiate of potash and alcohol, the latter into acetate of potash and wood-spirit. The differences between the several compounds produced by the action of sulphuric acid upon alcohol, viz. sulphovinic acid, ethionic acid, isethionic acid, &c. are probably dependent on metameric conditions. [For some examples adduced by Laurent, vid. Ann. Chim. Phys., 66, 175.] Aldehyde is C H 02, and its vapour weighs 1.5317; acetic ether, which is composed of acetic acid (C H3 O3) and ether (C H5 O1) is CH O1; and both its atomic weight and the spec. grav. of its vapour are twice as great as those of aldehyde. When cyanic acid is mixed in the cold with aqueous solution of ammonia, cyanate of ammonia is produced, as proved by the fact that the liquid yields cyanic acid when treated with sulphuric acid, and ammonia when treated with potash. But warming, or even spontaneous evaporation, is sufficient to convert the salt into urea, which does not exhibit these reactions with sulphuric acid and potash. Urea is C N H' O2; the |