A peculiar kind of isomorphism has recently been discovered by Scheerer (Pogg. 68, 319) which appears to play an important part in the mineral kingdom. By the analysis of a great number of minerals Scheerer finds that one atom of magnesia, protoxide of iron, or protoxide of manganese-and probably also of oxide of zinc, protoxide of nickel and prot oxide of cobalt-may be replaced by 3 atoms of water, and one atom of oxide of copper by two atoms of water-without change of crystalline form. This kind of isomorphism has received the name of Polymeric Isomorphism: it was first noticed in the minerals Cordierite and Aspasiolite. These two minerals crystallize in the same form, and crystals are found consisting partly of cordierite and partly of aspasiolite, the most complete transitions from one to the other occurring in the same specimen. Moreover, both minerals contain nearly the same proportions of silica and alumina; but aspasiolite contains a smaller quantity of magnesia and a larger quantity of water than condierite,-the difference being such that 3 atoms of water in the former may be regarded as the equivalent of 1 atom of magnesia in the latter. [Vid. Neumaun, Journ, für Prakt. Chem. 40, 1.]T On the subject of Isomorphism see the already (page 32) cited treatises of Mitscherlich, Beudant, Wollaston, Hauy and Marx; likewise Kobell (Schw. 64, 41).-Breithaupt (J. pr. Ch. 4, 249 and Pogg. 51, 510). Persoz (Ann. Chim. Phys. 60, 119; also Ann. Pharm. 18, 241).—Brooke (Phil. Mag. J. 12, 406).-Johnston (Phil. Mag. J. 12, 235 and 480; 13, 305).-Count Franz Schaffgotsch (Pogg. 48, 325). d. Relations to Heat. Fusibility.-Compounds are, for the most part, more easily fusible than their elements. No metallic alloy is more refractory than either of its constituent metals, but many are more easily fusible than either of their components in the separate state. The earths also become more fusible by combination. An alloy of nickel and platinum melts at the same temperature as copper; alloys of lead and tin, lead and bismuth, &c., melt more easily than either of those metals by itself. Iron, by combining with the infusible substance, carbon (in steel and cast-iron), becomes more fusible than it is in the pure state. Silica is not fusible in the blast-furnace, neither is lime, baryta, strontia, alumina, or magnesia, -but the combinations of silica with these bodies are unable to resist such a temperature. Many metallic sulphurets, on the contrary, are less fusible than their elements, e. g., KS, Zn S, Sn S, Hg S; others are less fusible than sulphur, but more fusible than the metal, e. g., Fe S, Ag S; but none of them are more fusible than sulphur. Why the melting point of a compound should be sometimes between those of its elements, sometimes below, and sometimes, though rarely, above them both, has not yet been explained. Volatility.-1. The elasticity of a compound is generally less than that of either of its elements. A solid or liquid may be formed by the combination of two gases, but no permanent gas is ever formed by the union of two liquid or solid bodies (p. 87). Phosphuret of nitrogen, when excluded from the air, will sustain a white heat without decomposition or volatilization, and even without fusion. 2. The volatility of a compound is very often of an intermediate degree; the more volatile element seems to impart a portion of its vola in t and we SO al a. a i " to the other. AFFINITY. Cachem boccanos gaseons by combining with oxygen, Very few compounds are more volatile than either of their con- want his st 46° C. (= 114-8° Fah.) Iese relations also have not yet been reduced to any regular law : this mach, b, wever, may be said, that a compound of given elements is for the most part less volatile the greater the number of atoms of which the ovaryonnà atom is made up. Sulphurous acid S O' is gaseous, sulnë kre gond SO is solid; the latter contains 1 atom more of the more volaThe Element, Axygen, but its total number of atoms is 4, that of sulpỪ 1.mous send only 3 1 At. nitrogen forms gaseous compounds with 1 are ? At errgen, liquid compounds with 3 or 4 atoms of that element. Orsinogen, CX, is gaseons, mellon, CN, solid, although it contains a grenzer proportion of the more volatile element. In some cases, however, the greater volatility of one of the elements more than compensates for the greater number of atoms in the compound; thus Fe2 Cl3 is more volatle than Fe Cl, thongh the former contains 5 atoms, the latter only two. For the specific heat of compounds vide Heat.] e. Relations to Light. ausparency.—A chemical compound is either transparent or opaque; ir the former case it transmits light, coloured perhaps, but always clearly, because chemically combined bodies refract light as a whole; turbidity always indicates mechanical mixture. Two transparent substances always form a transparent compound, two opaque ones an opaque compound; the compounds of transparent with opaque bodies are sometimes epaque, sometimes transparent. Oxygen combined with metals sometes forms transparent compounds, such as the alkalis and earths, oxide of zine, oxide of antimony, arsenious acid; sometimes opaque compounds, ... peroxide of manganese and magnetic iron ore: the compounds of sulphur with potassium, zinc, arsenic, and mercury are transparent, those with antimony, iron, copper, and silver, opaque. On the other hand, all the metallic fluorides, iodides, bromides, and chlorides appear to be transparent. According to this, fluorine, chlorine, bromine, and iodine would seem to have the greatest tendency to transparency, and oxygen a greater tendency than sulphur, inasmuch as the compounds of antimony, tellurium, and bismuth with oxygen are transparent, while their sulphurets are opaque. Among the metals those which show the smallest tendency to induce opacity in compounds, are the alkaline metals and arsenic; for all compounds of these metals with transparent substances are themselves transparent. Refractive Power-The refractive power of gaseous compounds is sometimes greater, sometimes less than the mean of the refractive powers of the constituent gases. This is shown by the following table of Dulong (Bullet. philomat., 1825, 132), which also contains the refractive powers of some of the simple gases. Column A contains the names of the gases;-B, their refractive powers determined by observation, that of air=1;-C, the active power which the compound gases should exhibit according to Aculation, taking the mean between the refractive powers of the component gases;-D, their specific gravities;-E, the specific refractive power, obtained by dividing the observed refractive power by the specific gravity. From these numbers Dulong concludes that when a compound gas is of an acid nature its refractive power is below the calculated mean, but above the mean when the gas is alkaline or neutral; hydrochloric ether, however, forms an exception to this rule. If it be admitted that the refractive power of a substance is directly proportional to its density and inflammability, the latter will be found by dividing the refractive power by the specific gravity. The quotients in column E agree very well with this view; hydrogen has the greatest refractive power, and oxygen, the substance most opposed in its properties to combustible bodies, the smallest. The other numbers also agree, excepting that the refractive power of sulphuret of carbon should be smaller than that of sulphuretted hydrogen, since the former contains 2 atoms of the less combustible substance sulphur to 1 atom of carbon, the latter equal numbers of atoms of sulphur and hydrogen; the refractive power of nitrous oxide, again, ought to be smaller than that of nitrogen, since nitrogen must lose some of its refracting power by combining with oxygen. Some compounds, in passing from the gaseous to the liquid state, increase in refracting power more than in density, as was first noticed by Arago and Petit. The absolute refractive power of liquid sulphurous acid ought, according to calculation from that of the gas to be 0 661: its actual value, however, is, according to De la Rive, 0.78 (Ann. Chim. Phys. 40, 410, extr. Pogg. 15, 528); that of liquid ammonia should by calculation from that of ammoniacal gas be 0.725, and that of sulphuretted hydrogen 0-767; but according to Faraday, the refractive powers of both these liquids exceed that of water, which is 0.784. tility to the other. Carbon becomes gaseous by combining with oxygen, hydrogen, or nitrogen; sulphur with oxygen or hydrogen-selenium, iodine, bromine, arsenic, antimony, and phosphorus, with hydrogen; but all these gaseous compounds are less permanent than pure oxygen, hydrogen, or nitrogen gas, for most of them may be liquefied by pressure. Lead, silver, iron, &c., in combination with chlorine, are volatile at a moderate red heat. 3. Very few compounds are more volatile than either of theirconstituents. The most striking instance is that of sulphuret of carbon, which boils at 46° C. (= 114·8° Fah.) These relations also have not yet been reduced to any regular law: thus much, however, may be said, that a compound of given elements is for the most part less volatile the greater the number of atoms of which the compound atom is made up. Sulphurous acid SO is gaseons, sulphuric acid S O3 is solid; the latter contains 1 atom more of the more volatile element, oxygen, but its total number of atoms is 4, that of sulphurous acid only 3. 1 At. nitrogen forms gaseous compounds with 1 and 2 At. oxygen, liquid compounds with 3 or 4 atoms of that element. Cyanogen, CN, is gaseous, mellon, C N, solid, although it contains a greater proportion of the more volatile element. In some cases, however, the greater volatility of one of the elements more than compensates for the greater number of atoms in the compound; thus Fe2 Cl3 is more volatile than Fe Cl, though the former contains 5 atoms, the latter only two. [For the specific heat of compounds vide Heat.] e. Relations to Light. Transparency.-A chemical compound is either transparent or opaque; in the former case it transmits light, coloured perhaps, but always clearly, because chemically combined bodies refract light as a whole; turbidity always indicates mechanical mixture. Two transparent substances always form a transparent compound, two opaque ones an opaque compound; the compounds of transparent with opaque bodies are sometimes opaque, sometimes transparent. Oxygen combined with metals sometimes forms transparent compounds, such as the alkalis and earths, oxide of zinc, oxide of antimony, arsenious acid; sometimes opaque compounds, e. g., peroxide of manganese and magnetic iron ore: the compounds of sulphur with potassium, zinc, arsenic, and mercury are transparent, those with antimony, iron, copper, and silver, opaque. On the other hand, all the metallic fluorides, iodides, bromides, and chlorides appear to be transparent. According to this, fluorine, chlorine, bromine, and iodine would seem to have the greatest tendency to transparency, and oxygen a greater tendency than sulphur, inasmuch as the compounds of antimony, tellu rium, and bismuth with oxygen are transparent, while their sulphurets are opaque. Among the metals those which show the smallest tendency to induce opacity in compounds, are the alkaline metals and arsenic; for all compounds of these metals with transparent substances are themselves transparent. Refractive Power. The refractive power of gaseous compounds is sometimes greater, sometimes less than the mean of the refractive powers of the constituent gases. This is shown by the following table of Dulong (Bullet. philomat., 1825, 132), which also contains the refractive powers of some of the Column A contains the names of the gases;-B, their refractive powers determined by observation, that of air=1;-C, the simple gases. duce acids when combined with oxygen; viz. hydrogen [?], carbon, ron, phosphorus, sulphur, selenium, bromine, iodine, nitrogen, silicium, tanium, tungsten, vanadium, chromium, uranium, manganese, arsenic, antimony, tin, and osmium. This law appears to fail in the case of mercury itself, which produces no acid, but is notwithstanding less volatile than corrosive sublimate. 2. All compounds containing 1, 3, 5 or 7 atoms of oxygen are either acid or basic; those on the contrary which contain 2 or 4 atoms of oxygen are, with a few exceptions which disappear on a particular hypothesis, neither acid nor basic. Exceptions to this law are-CO, which is neither acid nor basic, Mo O2, V Ô2, and Pt O2, which are bases, and CO2, S O2, Se O2, Ti O2, Sn O2, and Te O2, which are acids. The manner in which Persoz endeavours to get rid of these exceptions looks like a gratuitous assumption. It is nevertheless true that by far the greater number of acids and bases contain an uneven number of atoms of oxygen. The case in which marked chemical and physiological properties existing in elements are caused to disappear by combination is most strikingly exhibited in the combination of acids with salifiable bases: the effect is then called Neutralization. When an acid and a base combine in certain proportions, their opposite properties are mutually destroyed, and a more or less indifferent compound is the result. Hydrochloric acid, for example, tastes and smells strongly acid and reddens litmus; ammonia has a powerful alkaline smell and taste, restores the blue colour of litmus which has been reddened by an acid, reddens turmeric, and givesa green colour to violet juice-these changes of colour being again removable by acids; both these substances in the concentrated state exert a powerful caustic action on the animal body though in different ways, and cannot therefore be swallowed without injury, except in very small quantities and in a state of dilution. If now the aqueous solution of hydrochloric acid and aqueous solution of ammonia, be mixed in certain. proportions (the required proportions may be ascertained by the use of litmus or turmeric paper), a perfectly neutral compound will be obtained, which reddens neither litmus nor turmeric, tastes and smells neither acid nor alkaline, but is devoid of smell and has a saline taste, has no corrosive action, and may be swallowed in much larger quantities. The two substances have therefore neutralized each other both chemically and physiologically; a neutral compound has been formed; neutrality, chemical equilibrium, chemical indifference has been produced. The proportion in which this mutual destruction of chemical properties is most completely effected is called the point of neutralization. Exactly one atom of hydrochloric acid is required to neutralize one atom of ammonia. If to this neutral compound a fresh quantity of hydrochloric acid be added, the character of the acid will again become evident by its sour taste and its effect on litmus; it will prevail, preponderate, or be in excess, or the ammonia will be supersaturated with hydrochloric acid: similar results, only of the opposite character, would be obtained by adding more ammonia to the neutral compound. ADDENDA TO THE THEORY OF THE QUALITATIVE ALTERATION OF Although the properties of a compound are mainly dependent on those of its elements, and on the proportion in which these elements are combined, it has nevertheless been shown by recent experiments that other VOL. I. II |