salts of the stronger acids the distinction between water of constitution (basic water as it is sometimes called) and water of crystallization is, as a rule, well marked. Water of crystallization generally escapes at a comparatively low temperature, and the loss is attended with a complete disintegration of the crystals, which usually fall to powder. Water of constitution is held more firmly, and the salt must be heated to a much higher temperature before it is expelled. Moreover, although the loss of basic water may greatly alter the external aspect of the body, yet there is not the same complete breaking up of the structure as before. To illustrate these points, it is only necessary to refer to the well-known reactions of the common rhombic sodic phosphate when heated. It is generally believed that the water of crystallization exists as such in the salt, the molecules of water forming a part of the structure of the crystals; and the facts to be presented in this paper tend to support this view. But our modern theories assume that the so-called water of constitution, which may be also driven off when the salt is sufficiently heated, is formed under the influence of the heat from the atoms of hydroxyl, H-O-, which are united to the acid radical of the compound. But in every simple salt, whatever may be our theories in regard to the mode of combination, the weight of the water of constitution, as well as that of the water of crystallization, must bear to the weight of the salt a definite proportion, which can easily be calculated from the symbol; and the ratio between the amount of hydrogen and the amounts of the other radicals must be that of the atomic weights of these radicals or of their multiples. In any given class of salts, moreover, in which the ratio between the atomicity of the basic and acid radicals has a constant value, all hydrogen which represents water of constitution will supplement the other basic radicals, and added to them will complete the relative amount of basic radical which the given class of salts require; while the hydrogen which represents water of crystallization will be in excess of that amount. In mineral silicates these relations are complicated by the phenomena of isomorphous replacements; and although there may be some question in regard to the replacing capacity of hydrogen, we cannot expect the same constancy, in the amount of water of constitution which these minerals contain, as in the case of simple salts. The amount of water of crystallization, however, must be as invariable in one case as in the other, and the hydrogen must bear the same relations to the atomic ratio of the compound. Thus, if the mineral is an orthosilicate, in which the atomic ratio between the sum of the basic radicals and the silicon is 1:1, all hydrogen in excess of the amount which this ratio requires must represent water of crystallization, while all required to complete the ratio must represent water of constitution; and we thus have a means of distinguishing between these two states of combination where the class of compounds to which the mineral belongs is known. Now it is true of each of the minerals we have distinguished among the vermiculites, First. That the water is driven off at a temperature below a red heat. Secondly. That the loss of the water is attended with a complete disintegration of the mineral. Thirdly. That the amount of the basic radicals, exclusive of the hydrogen, is sufficient to saturate the silicon, and that the amount of hydrogen is wholly in excess of the amount which the atomic ratio 1:1 requires. Could it be proved that the vermiculites are orthosilicates, the last of the three facts just stated would be alone sufficient to establish the correctness of our conclusion. Unfortunately such absolute proof cannot be obtained; and we only claim that the crystallographic and chemical relations of the vermiculites to each other, and to the magnesian micas, give a very high degree of probability to our theory that they are orthosilicates. From these facts we have concluded that the water which enters into the composition of the vermiculites is water of crystallization. For the evidence of the facts, we refer to the descriptions of the several species given below. But, further, in order to justify our conclusion, we propose to bring into comparison with the vermiculites a class of hydrous micas from which the water obtained is clearly water of constitution; and this class of minerals we shall describe first. Damourite.-Delesse originally gave this name to a hydrous mica which occurs in fine scales in Pontivy in Brittany. Since then micas of similar composition have been observed in several countries, and shown to be not unusual constituents of granitic rocks*. Among these we may distinguish several varieties (or species ?), marked by slight differences of composition and optical characters. But we would propose to give the name Damourite to the whole class, distinguishing the varieties by separate names only so far as may be thought necessary. Under the family of Damourites, then, we class all unisilicate micas, which are chiefly silicates of aluminium and potassium, but in which a portion of the alkaline radical is replaced by hydrogen. Sterlingite.-A remarkable mineral of the Damourite type is found at Sterling, Mass., associated with spodumene, in a vein of a large boulder rock. This mineral, for the sake of distinc* See the papers of Professor Haughton, cited below. tion, I have called Sterlingite; but it does not differ from the original Damourite of Delesse except in the value of the optical angle. We give in parallel columns the characters of the two minerals: 10.86 1.84 11.20 10.90 1.85 5.25 4.50 4.00 99.52 100.00 Of the two analyses of Damourite, No. 1 is by Delesse, of the mineral found in the gangue of cyanite, at Pontivy in Brittany, and No. 2 is by Igelström, of the similar mineral found at Horrsjöberg, Wermland. The analysis of Sterlingite was made by Mr. C. E. Munroe, Assistant in the Chemical Laboratory of Harvard College; and the above numbers have been abundantly confirmed in repeated analyses by various students in the same laboratory. The alkalies were treated by Smith's process; and the potassium was weighed as PtK2 C16. This value, compared with the total weight of the alkaline chlorides and that of the chlorine, also determined, showed that the alkali in the mineral was almost wholly potash, although the presence of lithium and sodium was plainly indicated by the spectroscope. The water was determined by igniting the mineral in coarse powder, previously dried at 100° C. Even after ignition the finely pulverized mineral is only partially decomposed by hydrochloric acid; and in the above analysis it was decomposed by fusion with sodic carbonate. The usual tests failed to indicate the presence of fluorine. Regarding the water as basic, and as forming a part of the protoxides, the atomic ratio in Sterlingite between the silicon, the sesquioxide radicals, and the protoxide radicals, is IV VI 11 Si: R: R=23.40: 18: 6·45, or nearly 4:3: 1. The deviation from the simple ratio will not appear so great as scems at first sight, if it be noticed that a difference of one half per cent. in the amount of water would make the ratio almost exact. The corresponding ratio in the Damourite from Wermland is 23.15: 17.78: 6·41, or, as before, nearly 4:3: 1. These ratios point to the general formula II VI R, R. 08. Si2. The Damourite and Sterlingite are types of a very large class of hydrous micas, which in many places are widely distributed through the granitic rocks. This class of minerals has been especially investigated by Professor Haughton, of Dublin, whose papers may be found in Phil. Mag. S. 4. vol. ix. p. 272, and Quart. Journ. Geol. Soc. vol. xviii. p. 403, also vol. xx. p. 268. We cite here a few of his analyses, selected from those given by Professor Dana (pages 310 and 311 of the fifth edition of his 'System of Mineralogy') in further illustration of the subject we are discussing: Dublin Co.-Haughton. Optical angle, 53°. Si. Äl. Fe. Mg. Ća. Na. K. Ú. 43.47 31.42 4.79 1.13 1.38 1.44 10.71 5.43 99.77 23.18 14.64 1.44 0.45 0.39 0.37 1.82 4.83 Mount Leinster-Haughton. Optical angle, 72°. Glendalough-Haughton. Optical angle, 70°. In these micas again, if we regard the water as basic, we obtain a nearly constant ratio, but differing from that of Damourite in the relation of the two basic radicals. The general symbol of the last would be R6, R4.096. Sio. There appear to be, therefore, these two distinct types of hydrous micas related to the species Muscovite, all rich in alumina and alkali, destitute, or only containing very small amounts, of magnesia, and having a wide optical angle. The anhydrous Muscovites have not been investigated nearly as fully as the hydrous varieties; and I can find no analyses of any of the beautiful specimens from our American localities. I cite here, for the sake of comparison, an analysis of the Fuchsite, |