,3.0 GEOLOGICAL MEMOIRS. 



from aqueous solutions. I started upon a conviction that crystals 

 found in geodes could have been formed neither by evaporation nor 

 by refrigeration of saturated solutions, and I think I have succeeded 

 in discovering the mode of formation of such minerals. The method 

 is equally applicable to very soluble or slightly soluble bodies, and 

 admits of an infinite variety of modifications in its mechanism. Its 

 principle is nothing else than a gradual alteration of the affinity of 

 the solvent for the dissolved body, so that the precipitation occurs 

 very slowly. The gradual change of chemical force is obtained by 

 the diffusion of one liquid into another, such as in mixing produce a 

 solid precipitate. The arrangement of the apparatus is the same as 

 in Graham's experiments. Powdered chromate of potash was placed 

 in the bottom of a long glass cylinder, and powdered nitrate of lead 

 in the bottom of another ; both were then filled with water, and 

 placed in a large beaker-glass which contained water enough to cover 

 the two cylinders. In a few months the nitrate of lead had diffused 

 out into the beaker-glass and formed several beautiful amorphous 

 compounds on the edge of the cylinder in which the chromate had 

 been placed. In the interior of the cylinder, beautiful pink, highly 

 refractive needles of Rothbleierz (PbO, CrOg) were deposited, also 

 little dark red rhombic plates of Melanochroit (3PbO, 2Cr03). The 

 needles of neutral chromate found in this manner attained to three or 

 four millimetres, and then fell to the bottom of the cylinder, where 

 the conditions of their development were wanting. Had it not been 

 for this circumstance, they would, no doubt, in three or four months, 

 have got to half an inch, or even more. Some crystals of Weiss- 

 bleierz (PbO, CO2) formed in the same vessel, owing no doubt to the 

 circumstance that the chromate contained some carbonate of potash. 

 In a similar manner I obtained crystals of calcspar, also rhombic 

 plates of 2CaO, HO, PO5-I-4HO, and some shining needles which I 

 believe to be 3CaO, PO5. 



As this method is perfectly general in its principle and proves ap- 

 plicable to such compounds as carbonate and chromate of lead, we 

 may safely affirm that the insolubility of a compound will no longer 

 prevent its being prepared in a crystalline form. It appeared in these 

 experiments as if the great length of time which elapsed before the 

 crystals formed was owing to the salts not diffusing out rapidly enough; 

 I therefore modified the form of the experiment by placing a vessel 

 full of a dry salt inside a larger vessel containing a solution just 

 sufficient in quantity to cover the inner vessel. A large precipitate 

 formed on the undissolved salt, and in a few days little crystals were 

 perceptible in the amorphous mass, which continued to grow as long 

 as the materials lasted. In this way I hope to obtain good-sized 

 crystals of heavyspar, calcspar, sulphate of lead (Schwerbleierz), py- 

 romorphite (3 (PbO, PO^ + PbCl), apatite, &c. By diffusion of a 

 solution of silicate of potash into one of aluminate of potash I hope 

 to obtain felspar. The crystallization of very soluble compounds 

 may be accomplished by a similar process. Thus, if a solution of 

 sulphate of iron in a beaker-glass is covered with a thin stratum of 

 water, and alcohol gently poured on the top of that, a good and rapid 



