yields soda and hydrogen gas. The oxygen which was 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 formulæ 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 isomorphous 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.

dus, 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 O3 + 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

form of the prism, which is found, when broken, to con. sist of a mass of octohedrons."

234. The two or three forms are not, however, of equal stability. "One of the forms appears to be the stable condition of arrangement; the other forms appear to be produced by the sudden fixation of the molecules, in a form which is naturally only transitive, and from which they free themselves as soon as the external circumstances admit of their suitable motion amongst each other." A change of temperature is sufficient to produce this alteration in form. In some cases it can be brought about by mere rubbing; and in others it appears to be produced without the aid of any external agent.*

EXAMPLES.

1. When a prism of arragonite is heated in the flame of a spirit-lamp, it breaks up into a congeries of little rhombs of common calc-spar, at a temperature far below that at which the carbonate lime commences to be decomposed; but no alteration of temperature can convert calc-spar back again into arragonite. Indeed, arragonite appears to be formed only between very narrow limits of temperature. We have noticed (233) that at 212° F., artificially prepared carbonate lime takes this form, but below that point it takes the form of calc-spar; and we now see that at temperatures higher than 212° F., arragonite becomes converted into calc-spar.

2. Proto-iodide of mercury separates from solution, and likewise sublimes at a very gentle heat, in scarlet tables, belonging to the square prismatic system; but when

In solid bodies, a difference of molecular structure, fully equivalent to that to which dimorphism may be referred, is capable of being produced by very simple means. Thus, when a plate of glass is compressed by means of a screw, it assumes a doubly refracting structure, and gives, with polarized light, a cross, and rings, variously disposed, according to the direction of the pressure. In this case, the change of structure arises, necessarily, from an increase of density in the compressed portions; but the same effect may be produced by the converse process. A plate of glass which has been suddenly cooled from having been red-hot, assumes a similar double refracting and polarizing structure, although here the density is diminished, in place of being increased. I have found the specific gravity of glass, suddenly chilled, to be about less than that of glass of the same kind which had cooled slowly, indicating that the volume was the same that it had occupied at a dull red heat, and that hence the internal molecules were arranged so as to occupy a greater space than in the usual condition.-Sir R. Kane.

sublimed at a higher temperature, in sulphur-yellow rhombic tables, of the oblique prismatic system. The red crystals turn yellow as often as they are heated, and resume their red tint on cooling. The yellow crystals obtained by sublimation retain their colour when cooled; but on the slightest rubbing or stirring with a pointed instrument, the part which is touched turns scarlet; and this change of colour extends with a slight motion, as if the mass were alive, throughout the whole group of crystals, as far as they adhere together. In this case, the yellow crystals retain their external form unchanged, while the compound atoms must have taken up the relative position which belongs to the red crystals; the yellow crystals are therefore pseudomorphous. The same crystals turn yellow every time they are heated, and red again on cooling. The original red crystals also turn yellow when heated, and retain this colour, after cooling, for several days, even when touched with foreign bodies; and at length spontaneously, but very slowly, resume their red colour. When the red crystals are sublimed at a very gentle heat, red and yellow crystals sublime together. If a glass plate, having both red and yellow crystals on it, be warmed so gently that the red ones do not change colour, but sublimation, nevertheless, goes on, both red and yellow crystals collect on a plate held above the former. Now, since the upper plate is cooler than the lower, and the latter is not hot enough to change the colour of the red crystals, the yellow crystals in the upper plate can have only come from those of the same colour on the lower; they must, therefore, have sublimed as yellow crystals, and the vapour of the yellow crystals must be different from that of the red ones.

3. If melted sulphur be allowed to cool slowly, till a portion of it has become solid, and the still liquid portion be then poured out, the solidified portion exhibits oblique rhombic prisms, belonging to the oblique prismatic system. These are at first perfectly transparent, of a deep yellow colour, and somewhat harder and denser than those of sulphur crystallized in the cold; but after being kept for a few days at the ordinary temperature, they become opaque, of a straw-yellow colour, and become changed into a mass of very minute bright rhombic octohedrons.

4. When arsenious acid is crystallized in rhombic