182 
THE GEOLOGIST. 
tions under wliicli 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 Eose 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 cr^^stallized 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 gi^'en 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 Tuoths of an inch on the side. 
These various points are apparently trivial, but in their application they 
may be of considerable importance, as what aj^pear to be small things very 
often are. Gustavo 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 was formed. All depends upon the solution being very 
dilute and at the ordinary temperature. If a common solution of carbo- 
