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THE GEOLOGIST. 



tions under which it may be produced ; and, when we understand these 

 conditions, we shall find that the information afforded will tend to eluci- 

 date the formation of certain geological phenomena, especially with refer- 

 ence to temperature. It will indicate to us the temperature and other con- 

 ditions under which rocks containing arragonite may have been formed. 



Arragonite may be formed by dropping common chloride of calcium in 

 molten carbonate of potash or soda. The method is to melt the carbonate, 

 and then drop in the chloride of calcium. Decomposition occurs, and 

 chloride of potassium or sodium and carbonate of lime are formed. We 

 owe to Grustave Hose several experiments upon this subject, which, though 

 they may seem somewhat minute and tedious, are extremely important and 

 interesting. In the molten state the mass is clear, but it becomes opaque 

 and white on solidification. Upon washing the product with cold water, 

 an amorphous carbonate of lime or chalk was produced. It was always 

 obtained at first in minute microscopic globules, perfectly amorphous, or 

 non-crystalline ; but Eose tells us that after twenty-four hours the whole 

 became changed into rhombic crystals of calcspar. That is a curious point, 

 and the lecturer asked particular attention to the temperature of the water 

 employed, and the degree of dilution, for all depended upon those two con- 

 ditions. On the other hand, by boiling the product in water, instead of 

 washing it with cold water, the globules are almost instantly changed, 

 not into rhombs of calcspar, but into prisms of arragonite. This little dif- 

 ference of temperature, then, is sufficient to effect this great change. These 

 microscopic crystals of arragonite being left to cool in the water, become 

 transformed into rhombs of calcite. He found that the same results were ob- 

 tained by substituting chalk, arragonite powder, or calcspar, for chloride of 

 calcium. Some experiments on this subject were made a long time ago by 

 Becquerel, to whom we are deeply indebted for a great variety of experi- 

 ments bearing on the subject of mineralogy. He was one of the first to 

 take up the subject energetically. He formed arragonite by leaving plates of 

 selenite or gypsum during several years in contact with a solution of bicar- 

 bonate of soda of the specific gravity of 1*070. The result of the decomposi- 

 tion was sulphate of soda and carbonate of line. The carbonate of lime pro- 

 duced appeared in the form of crystallized arragonite. The crystals consisted 

 of very acute double pyramids, base to base, thus producing a very acute 

 dodecahedron. The same result was obtained in a few days by heating to 

 the boiling-point (100° C.) plates of selenite in a solution of bicarbonate of 

 soda, saturated cold. The solution was contained in hermetically-sealed 

 tubes of glass, and great pressure was given by a very ingenious artifice. 

 There was no necessity in this case for raising the temperature of the glass 

 very high to get the pressure. The pressure required was about five at- 

 mospheres, and this was obtained by half filling the glass with the solution 

 in question, and then putting in a few drops of bisulphide of carbon, which 

 is an exceedingly volatile body. It was inert, having no effect what- 

 ever upon the solution, but it enabled him to get a pressure of five atmo- 

 spheres at this low temperature. He tells us that the crystals of arrago- 

 nite which he thus formed were very distinct and very limpid, and in ten 

 days they were Tooths of an inch on the side. 



These various points are apparently trivial, but in their application they 

 may be of considerable importance, as what appear to be small things very 

 often are. Gustave Eose found that, by leaving a very dilute aqueous so- 

 lution of carbonate of lime in excess of carbonic acid, freely exposed to the 

 air, arragonite Mas formed. All depends upon the solution being very 

 dilute and at the ordinary temperature. If a common solution of carbo- 



