CHEMICAL SCIENCE. 297 



are forced to enter into the structure of crystals. He quoted the experi- 

 ments of Beudant, which he had repeated, in which Prussian blue was 

 included in crystals of nitre and alum. He also alluded to the effects of 

 different menstrua, in modifying the forms of crystals or totally changing 

 their forms, instancing the crystallization of sea salt in the forms of the 

 regular octahedron in a solution of urea, whereas the cube is its usual 

 form. 



Dr. A. A. Hayes followed Dr. Jackson. Having inquired of Dr. J. how 

 large was the proportion of sand in the Fontainblcau crystallized sandstone, 

 and received for answer about fifty per cent., he said that he had often ex- 

 amined the spots where they were forming, and had noticed a growth equal 

 to the size of a garden bean, to take place in the course of two or three weeks 

 of wet, spring-time weather. To form a just conception of the conditions, 

 the fact must be kept in view, that the beds containing them arc composed 

 of fine silts, and in the case immediately under view, these were arranged in 

 plane.? of deposition of alternate courses, covered by much finer material, in 

 layers of different thickness; so that the mass was stratified; the coarser 

 layers being very permeable to water. The rounded forms, often strongly 

 resembling organic remains, are found resting between these layers, and a 

 condition necessary to their formation is, the presence in the layer or rock 

 above them of abundance of carbonate of lime. 



The force exerted by some salts in their tendency to crystallize is brought 

 into view only when we study their formation, and carbonate of lime is one 

 of the constantly -occurring salts which well illustrates, in a remarkable 

 manner, this power of assuming regular forms. As lias been stated, with 

 fifty per cent, of its weight of sand, it forms regular rhomboids, but the more 

 recent observations of some African travellers, who found their progress im- 

 peded by "stone plants," six or eight inches high, formed of aggregates of 

 spear-shaped crystals of sand, cemented by carbonate of lime, show, that 

 this large proportion may be exceeded, while the foreign material is in a 

 somewhat coarse state. 



In the formation of claystones, however, we are to consider the presence 

 of finely-divided matter suspended in, or so mixed with water of infiltration 

 in spring-time, or general saturation from position, that it has nearly a semi- 

 fluid state. A saturated solution of bicarbonate, or more commonly crenate 

 of lime, finds its way into the soft mass, by frost crevices, or channels left by 

 roots, or even air-bubbles, and at these points the concretions commence, 

 when no nuclei of similar chemical composition exist. The finely-divided 

 matter interposes an obstacle to the formation of crystals of carbonate of 

 lime, far greater than an equal amount of coarser foreign matter would do ; 

 and we observe, then, the influence of that beautiful law in accordance with 

 which rounded forms are produced. In the laboratory similar forms daily 

 occur, where the presence of finely-divided and diffused bodies arrests the 

 formation of crystals, and globular, or curved-surfaced solids are produced; 

 as in the animal frame, the cell-structure causes the dissolved phosphate of 

 lime to take the curvilinear form pertaining to organization. The claystones 

 which are produced under the simple conditions here described, have no cou- 



