equal; and an atom of any electrolyte requires for its decomposition the same quantity of electricity as an atom of any other. Thus the quantity of electricity which will decompose one atom of water, will decompose one atom of chloride of lead, one of oxide of silver, or one atom of any electrolyte. Now, on decomposing an oxysalt of the alkalies or alkaline earths,-say sulphate of soda,-for every atom of water, or chloride of lead, which is decomposed in the voltameter placed in the course of the circuit, one atom of S O1, and one atom of oxygen, accumulates at one electrode (the zincode), whilst one atom of soda, and one of hydrogen, is set free at the other electrode (the platinode); yet we have stated that it is a fundamental law of voltaic action, that the amount of force circulating in any circuit at the same time, is equal in every vertical section of the circuit; and, consequently, its decomposing energy in each section must also be equal. Also that an atom of any electrolyte requires for its decomposition the same quantity of electricity as an atom of any other; yet for every atom of the electrolyte decomposed in the voltameter, we have one equivalent of sulphate of soda, and one equivalent of water, decomposed. Now, on the binary theory, sulphate of soda is not composed of soda and SO,, but sodium and sulphion (SO); the sodium, therefore, is liberated at the platinode, whilst sulphion is liberated at the zincode. But the sodium cannot exist in a free state in the presence of water, and sulphion cannot exist free; both the simple and compound radical, therefore, decompose water as soon as they are liberated; the sodium combines with the oxygen, setting the hydrogen free, whilst the sulphion combines with hydrogen, setting the oxygen free: therefore, hydrogen is liberated along with soda at the platinode, and oxygen is liberated along with sulphionide of hydrogen at the zincode; and in equivalent proportions, because an atom of water is decomposed by every liberated atom of sodium, and an atom of water by every liberated atom of sulphion. According to this explanation, only one atom of one electrolyte sulphionide of sodium is decomposed, for each atom of the electrolyte which is decomposed in the voltameter, as the decomposition of the water is occasioned, as we have seen, by a secondary action. If the metal of the salt is one which does not decompose water at ordinary

temperatures, then no hydrogen, but only the metal in its uncombined state, is set free at the platinode, whilst sulphionide of hydrogen and oxygen are found at the zincode, owing to the sulphion decomposing water.*

226. Kopp considered that the binary theory of salts was supported by the atomic volume of salts, but Filhol has shown that the experimental results may be explained quite as well by the older view of the constitution of salts; we shall, therefore, defer noticing Kopp's views until we come to consider the atomic volume of solids.

227. Both views of the constitution of salts are hypothetical, and although many chemical changes can be explained, as we have seen, in a more simple and philosophical manner by the binary view than by the old one, yet there are many objections to the binary theory, and it will, probably, give place to other and still newer views of the constitution of salts, which we shall presently bring before the attention of the student. Our objection to the salt-radical theory is the necessity of creating a great number of compound radicals, which never have and probably never will be isolated, such as SO, NO, CO,, &c. Another objection Professor Miller thus states:-"It appears to be highly improbable that a body of such powerful affinities as potash should, in carbonate of potash for example, part with its oxygen to a substance which, like carbonic acid, exhibits no tendency to further oxidation, so that K O, CO,, should become K, CO,."

228. Hess observes on Daniell's theory,-"The theory which regards sulphate of soda as Na S O., certainly affords the simplest explanation of its electrolysis. Since, however, many weighty reasons may be urged against the adoption of this hypothesis, the following explanation may for the present be admitted. Sulphate of soda is Na O, SO,; decomposition by the electric current is exerted only on the soda (since, by Faraday's law,† SO, is incapable of direct decomposition). Sodium separates at the negative pole, where it decomposes water and

The student who desires to pursue the subject further is referred to the second part of Miller's "Elements of Chemistry," or to the original memoirs on this subject by Drs. Daniell and Miller, in the "Phil. Trans." 1844. † See the Author's "First Step in Chemistry," 3rd edition, p. 177.

The oxygen

which was

yields soda and hydrogen gas. combined with the sodium is transferred, together with the sulphuric acid, to the adjacent atom of sodium. An atom of oxygen is set free at the positive pole; and since the sodium which was combined with it goes towards the negative pole, the sulphuric acid is set free by secondary action, or rather it passes from its state of combination with soda into that of combination with water. In the case of sulphate of copper, &c., similar actions take place, excepting that the metal liberated by the current does not decompose water.

229. The formula of the following salts must now be written out in accordance with the binary theory.

EXERCISES.

110. Bisulphate of soda. 111. Sulphate of alumina. 112. Nitrate of ammonia. 113. Chlorate of potash. 114. Bromate of soda.

115. Sulphate of potash and magnesia.

116. Phosphate of silver (monobasic phosphate). 117. Phosphate of alumina

118. Phosphate of silver (bibasic phosphate). 119. Phosphate of alumina

120. Phosphate of silver (tribasic phosphate). 121. Phosphate of alumina

134

CHAPTER V.

POLYMORPHISM, PSEUDOMORPHISM, ISOMORPHISM.

Dimorphism, 230. Trimorphism, 231. Influence of heat in causing bodies to crystallize in one system or another, 233. The different crystalline forms of the same substance not of equal stability, 234. Examples. Isodimorphism, 235. Examples. The same substance in its different crystalline forms differs in its physical characters, 236. Examples. List of dimorphous bodies, 237. Pseudomorphism, 238. The processes by which pseudomorphs are produced, 240. Isomorphism, 243. Behaviour of two or more non-isomorphous crystalline substances in the same solution during crystallization, 244. Behaviour of two or more isomorphous crystalline substances in solution during crystallization, 246. Intermixture of isomorphous substances in minerals, 249. Table of some of the most important groups of isomor phous substances, 251. Graham's opinion upon the isomorphous relations of water, 252. Scheerer's view upon the isomorphous relations of water, 253. Polymeric isomorphism, 254. The method for deducing the rational formula for compounds containing isomorphous constituents, 255. System of notation employed in mineralogical works, 259. Exercises. The different reasons which have been suggested in explanation of the phenomena of isomorphism, 261. Kopp's view that isomorphous bodies have the same equivalent volume, 263. Some of the views held upon bodies with like forms but unlike constitutions, 265. The chief points which have been brought before the notice of the student, 266. Aid derived from isomorphism in determining equiva lents, 268.

230. POLYMORPHISM.-Some elementary and compound bodies are capable of assuming two distinct crystalline forms; substances which can thus crystallize, according to two different systems, are called dimorphous (from

*

The names and definitions of the six systems into which crystals are subdivided, are given at p. 142.

ds, twice, and poppǹ, shape), and the phenomenon itself has received the name of dimorphism. Example.-The crystals of carbonate of lime in calcareous spar and in arragonite belong to different systems of crystallization.

231. Some substances are even trimorphous, that is, they crystallize in three different systems. Both the seleniate of zinc (Zn O, Se O, +7 aq.), and sulphate of zinc (Zn O, SO, +7 aq.), and the seleniate of nickel (Ni O, Se 0, +7 aq.), and sulphate of nickel (Ni O, SO, +7 aq.), according to Mitscherlich, exhibit this peculiarity.

232. Laurent and Pasteur have observed that the forms of dimorphous crystals border upon the limits of the two systems, and under the influence of certain determining conditions, can pass easily out of the one into the other system.

233. Whether a body shall crystallize in one system or another seems to depend chiefly on temperature. Examples.-Carbonate of lime artificially prepared takes the form of calc-spar or arragonite, according as it is precipitated at the temperature of the air or near the boiling point of water. Sulphur, in crystallizing from solution in bisulphide of carbon or in oil of turpentine, at a temperature under 100° F., forms octohedrons with rhombic bases, but when melted by itself and allowed to cool slowly, it assumes the form of an oblique rhombic prism on solidifying at 232° F. Crystals formed at one particular temperature, and then exposed to that temperature at which the substance assumes its other crystalline form, frequently become changed, without alteration of external form, into an aggregate of small crystals of the latter kind; the change from one form to the other is often attended with a change in the crystals from transparency to opaqueness. Example.-We have already noticed that sulphate of nickel crystallizes in three forms ; "it crystallizes below 59° F. in right rhombic prisms; between 59° and 68° in acute squarebased octohedrons; and when the temperature is above 86° in oblique rhombic prisms. In the first case, the crystals belong to the prismatic; in the second to the pyramidal, and in the third to the oblique system. If the right rhombic crystals be placed in the summer's sun for a few days they become opaque, but still retain the