OEIGIN OF CEYSTALLINE EOCKS. 85 



bonates. A solutiou of chlorid of maguesium, holding- some chlorid of sodium and sulphate 

 of magnesia, in like manner dissolved 1.00 gramme of carbonate of lime to the litre. Such 

 solutions have an alkaline reaction." 



These sokitions, which contained, in all cases, neutral carbonates, with no excess of 

 carbonic acid, possessed a considerable degree of stability. One prepared with 0.80 

 gramme of carbonate of lime and 1.00 gramme of carbonate of magnesia, when filtered after 

 standing eighteen hours at 10' C, still retained 0.t2 gramme of carbonate of lime to the 

 litre, but, after some days, deposited the whole of this in transparent crystals of hydrous 

 carbonate of lime, all of the carbonate of magnesia remaining dissolved. This hydrous car- 

 bonate, stable at low temperatures, is at once decomposed, with loss of its water, at 30' 0. 

 " The solubility of the yet uncondeused carbonate of lime in neutral solutions, which are 

 without action upon it in another state of aggregation, is a good example of the modified 

 relations presented by bodies in the so-called nascent state, which probably, as in this case, 

 consists of a simpler and less condensed molecule. At the same time, the gradual spontane- 

 ous decomposition of the solutions thus obtained affords an instructive instance of the 

 influence of time on chemical changes." "** 



§ 111. The spontaneous conversion of uncrystalline precipitates into crystalline aggre- 

 gations may next be noticed. Instances of this are well known to chemists, but a remark- 

 able and hitherto iindescribed example is afforded in the case of the mixed oxalates of the 

 cerium-metals, got by precipitating their nitric solution with oxalic acid in the cold. A 

 tough pitchy mass was thus repeatedly obtained which, in a few minutes, changed into 

 incoherent crystalline grains, the conversion being attended with a notable evolution of 

 heat. Another example of a somewhat similar phenomenon is presented in the case of 

 the amorphous insoluble malate of lead, which, as is well known, spontaneously changes 

 into crystals beneath the liquid in which it has been precii:)itated. 



§ 112. In the paper above quoted on the salts of lime and magnesia, I have described 

 not less remarkable examples of similar transformations in the case of the carbonates of 

 lime and magnesia. A paste of hydrous carbonate of magnesia precipitated in the cold, 

 slowly changes under water, at ordinary temperatïires, into a crystalline mass made up of 

 prisms, grouped in spherical aggregations, of the well-known terhydrated magnesian carbo- 

 nate. In like manner, the amorphous paste got by triturating in a mortar a solution 

 ofchlorids of calcium and magnesium, in equivalent proportions, with the requisite 

 amount of a solution of neutral carbonate of soda is, at a temperature of from 65° to 80" 

 C, changed, after a few hours, into an aggregate of translucent crystalline spheres of a 

 hydrous double carbonate, resembling the hydrodolomite of von Kobell. At temperatures 

 of from 15° to 18° C, the same magma changes slowly into a more highly hydrated com- 

 pound. The process of change, which requires from twelve to twenty-five days, appeared 

 " to consist in the formation of nuclei from which crystallization proceeded until every 

 particle of the once voluminous, opaque, and amorphous precipitate had become translucent, 

 dense, and crystalline." The product is made up of brilliant prisms, apparently oblique, 

 grouped around centres, and sometimes forming spheres five or six millimetres in diameter. 

 The hydrated double carbonate of lime and magnesia, thus formed in presence of a 



""Hunt, Contributions to tlie History of Lime and Magnesia Salts; Part ii., 1866. Amer. Jour. Science, vol. 

 ^lii., pp. 58, 59. 



