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35. 3Be 0, 2 Al O2, 5Si 0 [?] (Euklase),—3Ca 0, 2Alo 0%, 5Si O’ [?] (Zoisite). Brooke.

One-and-one-membered or Doubly Oblique Prismatic System. 36. MnO, SO?, 4Aq,-Mn 0, Se 0°, 4Aq,-Zn 0, Se 0°, 4A9,-Co O, Se 0", 4 Aq. Mt.

37. Cu O, SO', 5Aq,-Cu 0, Se 0, 5 Aq,-Mn0, SO', 5Aq. (fig. 121, 122, 123). Mt. 38. a. Na 0, 3Ca 0, 4Al' 0", 12Si 0 (Labrador). b. Mg 0, 3Ca 0, 4A1 09, 8Si 0 (Anorthite).

Six-membered or Rhombohedral System. 39. 3Ag S, Ass (Light red silver).-3Ag S, Sb S' (Dark red silver).

40. a. Ca 0, C 0 (Calcspar),—Mg0, C 0,Ca 0, c0 + Mg0, C0,--MnO, C0,-ZnO, CO-Fe 0, Co (fig. 141 and f.)

b. Na O, N 0°,-KO, N O'. Frankenheim. 41. a. Si O(Quartz).—6. Ca 0, Al? 0, 4Si 0°, 6Aq. (Chabasite). 42. Al O' (Corundum),-Fe0,-Cr 0',-Fe Ti 0 (Ilmenite).

43. As,-Sb,—Te. Breithaupt. Tin also, according to Breithaupt, and zinc, according to Nöggerath, crystallize in regular six-sided prisms.

44. Sr O, SO, 4A9,--Ca 0, SP 0°, 4Aq,-Pb O, SO', 4Aq. Heeren.

45. Ca CI, 9Ca 0, 3P 0(Apatite),—PbCI, 9PbO, 3PO: (Grünbleierz), -Pb CI, 9Pb 0, 3As 0%.

46. Mohsite and Eudialyte. Brooke.

47. a. Cd S,—Ni S,-Fe* S'.—6. Ni’ As (Kupfernickel),—Ni Sb Sa (Nickeliferous grey antiinony).

c. Ir Os. Breithaupt. 48. H O and Zn 0, crystallize in regular six-sided prisms, but their isomorphism has not yet been established by the determination of their angular relations.

From these data the isomorphism of the following simple and compound bodies may be deduced :

Carbon, phosphorus, potassium, titanium, bismuth, cadmium, lead, iron, copper, silver and gold? (1, a)

The isomorphism of iron and titanium is more completely established by (42) inasmuch as Fe: 0' and Fe Ti 0 crystallize in the same form, and that of iron and cobalt by (25).

Potassium, sodium, lithium, calcium, zinc, lead, silver? (1,6)
Oxygen, sulphur, chlorine! (1,c)
Arsenic, antimony and tellurium (1, d, 13, 39, 43).
Platinum, iridium aud osmium (1,9).

One atom of arsenic probably replaces two atoms of sulphur, inasmuch as Fe’ S' and Co? As S have the same form (2); so likewise Fe' St and Fe’ As S (14).

Potassium and ammonium, NH(1, b and 9,417,6).

K O and N H 0; also under certain circumstances Na 0 (1, 2,-5). On the other hand we might admit from (19) that NHO + H O replaces KO,

Na 0 and Ag O (18, a). Just as K O in alum is isomorphous with Na 0, so likewise in combination with N O it may under peculiar circumstances crystallize in obtuse rhombohedrons exactly like those of Na 0, NO: (41), so that NHO, KO, Na O, and Ag 0, may be regarded as isomorphous in one or other of their states.

MgO, MnO, ZnO, Fe 0, Co O, Ni O and Cu 0, have been shown to

Cr 03 seems

5ši 03 [!e isomorphous in their combinations with carbonio, sulphuric and selenio

'acids (1, 7,-4,-20,-22-29,—30,—36). Lime, Ca 0, is also related to them in one of its dimorphous states.

Pb O, Ba O, Sr 0, and Ca 0 (in one of its states) are isomorphous (1,5,-3,-16,-17,-24,-44,-45). According to (35), if the formulæ are correct, Be O is isomorphous with

According to (23), we might suppose that Na O is interchangeable with Ca 0, HO.

AlO', Cr? 0% and Fe’O), are isomorphous in the separate state (42); Mn0is also isomorphous with them in combination (1, 1,-1, h).

Ti O and Sn 0 are isomorphous in the separate state, (9) although Ti and Sn crystallize in different forms.

WO in combination is isomorphous with Mo 0% and Cr 03 (3), also with Ta 03 (27).

S 03 in combination is isomorphous with Se O’, Cr 0”, and Mn 03 (4,-7,-18, ,-19,20,-26, 6, -28,-29,-36,-37). also to form a connecting link of the series W 09, Mo 03, Cr 0}, Mn 03, Se 0", and S 0%.

PO' and As Os are also isomorphous in combination (5,-21,-32,33,-35).

CI O’ in combination is isomorphous with Mn’O (17, 6).

The following similarly formed compounds, however, díffer so much in their chemical properties that their similarity of shape can scarcely be regarded as resulting from the substitution of one element for another:

Pb 0, N 03 and Pb O NOʻ, (1,5) have the same form though they differ in composition by 2 At. oxygen.

Leucite and Analcime have the same composition, (1, 1) excepting that tbe latter contains 2 atoms of water.

Copper-pyrites Cu Fes? and Braunite Mn 03 (8). Here it must be supposed that 3 atoms of oxygen may be substituted for 2 atoms of sulphur.

Anatase and Apophyllite (10).-Zircon and Wernerite (11).-Manganite and Prehnite (15);-all differing irreconcilably in their chemical composition.

Arragonite, Ca 0, CO and nitre KO, NOS (16).'

Ba 0, SO on the one hand, and K 0, Cl 07 and K 0, 2Mn 07 on the other (17)

Na 0,'S 0' and Ba 0, MnO (18).

Sb S3 and MgO, SO, 7Aq (20).-S and KO, 28 O, HO (26). Borax, Na 0, 2B 09, 10Ag and Augite, Cu 0, Mg 0, 2Si 02 (34).–Labrador and Anorthite (38).

Lastly, of totally dissimilar composition are: CaO, CO (Calcspar) and NaO, NOS (40). - Quartz and Chabasite (41). — Mohsite and Eudialyte.

Attempts have been made to bring some of these cases in accordance with the theory of isomorphism by altering the atomic weights of some of the substances concerned. The following is one of the most remarkable instances of this kind : Ca 0, COP as arragonite is isomorphous with nitre (KO, N 0) in its usual form (16); Ca 0, CO', as calcspar, with KO N O as it is sometimes obtained, and with Na O, NOS as it always crystallizes. Hence Ca 0, CO in its two conditions is isomorphous with KO, N Os in its two conditions. For this reason Count Schaffgotsch halves the atomic weights of potassium and nitrogen; nitre then becomes K0), N04 = KNO'. This agrees with Ca O, CO2 = Ca C03;

in both compounds 3 atoms of oxygen are combined with 1 atom of metal and 1 atom of either carbon or nitrogen. This halving of the atomic weight of potassium involves the halving of those of ammonium (N H), sodium, silver and gold, because potash is isomorphous with oxide of ammonium (N H'O) and soda, and the last of these with oxide of silver; and because silver in combination with the most various quantities of gold always crystallizes in the same form, a circumstance which indicates the isomorphism of these two metals. The halving of the atomic weight of silver had before been proposed by H. Rose, because in grey copper ore 1 atom of silver takes the place of 2 atoms of copper, and the crystalline form of Ags as well as that of Cu’S belongs to the regular system. According to this view the atomic weights of N, H, K, Na, Ag, and Au would be reduced to one-half of the values now assigned to them; potash would be K2 O, soda Na’0, oxide of silver Ag? 0, and suboxide of silver Ago. By halving the atomic weights of potassium and chlorine the similar forms of KÖ, CI O and Ba 0, S 0% would also be explained, (17) for the composition would then be K CI O' and Ba S O'.

Clarke, on the contrary, doubles the atomic weights of sodium and silver in order to reconcile the composition of Na O, S 03 and Ag0, S 03 with that of Ba O, Mn’ 07 (18). He thus obtains Na O’, 2S 03 = Na S208 and Ag O’, 2S 03 = Ag S?O"; and this formula agrees with Ba 0, Mn 07 = Ba Mn’O*. But since the atomic weight of potassium must be doubled as well as that of sodium, the explanation of case (18) becomes by this alteration more difficult than before.

With regard to these attempts, we cannot but agree with the view recently adopted by Johnstone (who formerly made trial of the same hypothesis as Count Schaffgotsch), viz. that many of the formulæ of isomorphous bodies cannot be made to agree with one another in any way whatever —others only by means of hypotheses which are either contradictory or greatly impair the simplicity of the chemical formulæ. Although similar formulæ often involve similarity of shape, it does not by any means follow that similar forms are inconsistent with dissimilar formulæ. There exists perhaps a higher law by which these cases might be explained: the discovery of such a law would give a new form to the theory of isomorphism.

Kopp and Schröder have remarked that isomorphous substances have equal atomic volumes (and therefore also equal atomic numbers). The simple substances (Table, page 55) exhibit

approximations to this law, at least in some cases: e.g. Ni, Mn, Co and Fe; W and Mo; I, Br and Cl. But the atomic numbers of Sb and As, of Na and K, of Mn, Cr, S and Se, which at least are isomorphous in their acids, differ widely. According to table (page 68) equal atomic numbers are exhibited by Al, Cr03 and Fe* 0°; by Ti Oo and Sn 0?; by W 03 and Mo 0°; by As 03 and Sb 0%; by several anhydrous carbonates, sulphates and nitrates (some too not isomorphous); and by several hydrated sulphates. Exceptions are however presented by KO, SO3 and KO, Cr 03; and by K CI, Na Cl and Ag CI; moreover the merely approximate agreement in the atomic numbers of the first named compounds may in a great measure be explained by the fact that similar formulæ give nearly equal atomic weights, and these being used as divisors of nearly equal specific gravities, the quotients cannot differ much from another, On the other hand, Kopp has shown that the small differences between the angles at the edges of the obtuse rhombohedron of calcspar and the corresponding angles in the other carbonates which are isomorphous with it, probably bear a simple relation to the different volumes of the atoms of which these crystals are composed.

TA 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. Neumann, 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, 335).

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

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 S O is gaseons, sulphuric acid S 03 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. i 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, CN, 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 Fe? Cl' 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, 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 of the constituent gases.

powers 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

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