30 RAINEY, ON CARBONATE OF LIME GLOBULES. 



The cause of these modifications of form produced by the 

 diflerent proportions of gum in combination with the earthy- 

 constituents, seems deducible from the facts already men- 

 tionedj namely, that these elements become intimately mixed 

 together in inconceivably minute quantities, and that the 

 gum in the mixture is rendered insoluble, so that the par- 

 ticles so formed and their elements being thus combined, 

 would be under the joint influence of the forces which each 

 element by itself would have been acted upon, and by which 

 its form would be determined. The particles of pure carbonate 

 of lime, being under a force which disposes them in straight 

 lines, would take the crystalline form ; whilst those of gum or 

 albumen, in which the tendency to attract one another is 

 probably strong, as indicated by their tenacity, would, by 

 their mutual attraction, be brought into the globular form. 

 Hence, in the mixture composed of the carbonate element in 

 great excess, the crystalline form would prevail, whilst in 

 that in which the viscid element preponderated the globular 

 form would predominate; and of course intermediate forms 

 would result from such diflFerent proportions of these elements 

 as might between these extremes. Some compounds formed 

 in gum do not become globular, as, for instance, oxalate of 

 lime. It remains beautifully crystalline, and increases by 

 the addition of fresh invisible particles to the surface of the 

 crystals, just as the globules do by the addition of particles 

 of carbonate and gum to their surface. This probably arises 

 from the oxalate not combining with the viscid substance and 

 solidifying it in the manner that the carbonate does. If the 

 particles of carbonate become deposited on a crystal of oxalate 

 of lime, they coat it with a globular, and not with a crystal- 

 line, layer, so that it would appear that these particles in their 

 very earliest state are spherical. Besides, the fact of the par- 

 ticles of carbonate thus combined with gum, when so small 

 as only just to be visible with the highest magnifying powers, 

 being also spherical, is in favour of this conclusion. ALL the 

 globular forms of carbonate of lime are considered by some 

 to be crystalline, and are called globular crystals. In some 

 of these forms there is a slight appearance of a crystalline 

 structure ; in others, where either the gum is small in quantity 

 or where the form of carbonate is mixed with some other 

 crystalline compound, or is a carbonate of lime of a more 

 highly crystalline character, this appearance is more strongly 

 marked; whilst there are those carbonate globules com- 

 bined with so large a proportion of gum as to present no 

 appearance whatever of crystallization, which, notwith- 

 standing, exhibit a distinct cross under polarized light. Now, 



