Page:On the various forces of nature and their relations to each other.djvu/66

know what common salt is^[1]: here is a piece of this salt which by natural circumstances has had its particles so brought together that they Fig. 15.⁠Fig. 16.⁠Fig. 17.⁠have been allowed free opportunity of combining or coalescing; and you shall see what happens if I take this piece of salt and break it. It does not break as flint did, or as the mica did, but with a clean sharp angle and exact surfaces, beautiful and glittering as diamonds [breaking it by gentle blows with a hammer]; there is a square prism which I may break up into a square cube. You see these fragments are all square one side may be longer than the other, but they will only split up so as to form square or oblong pieces with cubical sides. Now, I go a little further, and I find another stone (fig. 17) [Iceland, or calc-spar]^[2],

1. Common salt, or chloride of sodium, crystallises in the form of solid cubes, which, aggregates together, form a mass, which may be broken up into the separate cubes.
2. Iceland or Calc Spar.—Native carbonate of lime in its primitive crystalline form.