We also find compounds which have two distinct systems of crystallisation. This property, Dimorphism, is very strikingly shown in carbonate of lime, which occurs in rhombohedrons, in calc spar, and in rhombic prisms in arragonite. The molecular arrangements here are not, however, of equal stability, and one form is evidently forced upon the other, and is abandoned by it on the slightest disturbance. When a prism of arragonite is heated it breaks up into the rhombs of common calc spar, at a temperature far below that at which the carbonate of lime is decomposed; but no alteration of temperature can convert calc spar into arragonite. Crystals are found in the most microscopic character, and of an exceedingly large size. A crystal of quartz at Milan is three feet and a quarter long, and five feet and a half in circumference, and its weight is 870 pounds. Beryls have been found in New Hampshire measuring four feet in length.[38] In the dark recesses of the earth, where the influences which produce organisation and life cease to act, a creative spirit still pursues its never-ending task of giving form to matter. The science of crystallogeny,[39] embracing the theoretical and practical question of the causes producing these geometric forms, has in various ways attempted to explain the laws according to which molecules arrange themselves on molecules in perfect order, giving rise to a rigidly correct system of architecture. But it cannot be said that any theory yet propounded is sufficiently exact to embrace the whole of the known phenomena, and the questions,--What is crystallogenic attraction, and what is the physical nature of the ultimate particles of matter,--are still open for the inquiries of that genius which delights in wrestling with the secrets of nature. The great Epicurus speculated on the “plastic nature” of atoms, and attributed to this _nature_ the power they possess of arranging themselves into symmetric forms. Modern philosophers satisfy themselves with attraction, and, reasoning from analogy, imagine that each atom has a polar system.