figure when put back again into the solution of alum, which shows that the internal and external particles are similar, and have a similar attraction for the particles held in solution. The original conditions of aggregation which make the molecules of the same substance unite in different forms must be very numerous, since of carbonate of lime alone there are many hundred varieties; and certain it is, from the motion of polarised light through rock crystal, that a very different arrangement of particles is requisite to produce an extremely small change in external form. A variety of substances in crystallising combine chemically with a certain portion of water which in a dry state forms an essential part of their crystals, and, according to the experiments of MM. Haidinger and Mitscherlich, seems in some cases to give the peculiar determination to their constituent molecules. These gentlemen have observed that the same substance crystallising at different temperatures unites with different quantities of water and assumes a corresponding variety of forms. Seleniate of zinc, for example, unites with three different portions of water, and assumes three different forms, according as its temperature in the act of crystallising is hot, lukewarm, or cold. Sulphate of soda also, which crystallises at 90° of Fahrenheit without water of crystallisation, combines with water at the ordinary temperature, and takes a different form. Heat appears to have a great influence on the phenomena of crystallisation, not only when the particles of matter are free, but even when firmly united, for it dissolves their union, and gives them another determination. Professor Mitscherlich found that prismatic crystals of sulphate of nickel (N. 164), exposed to a summer’s sun in a close vessel, had their internal structure so completely altered without any exterior change, that when broken open they were composed internally of octahedrons with square bases. The original aggregation of the internal particles had been dissolved, and a disposition given to arrange themselves in a crystalline form. Crystals of sulphate of magnesia and of sulphate of zinc, gradually heated in alcohol till it boils, lose their transparency by degrees, and when opened are found to consist of innumerable minute crystals totally different in form from the whole crystals; and prismatic crystals of zinc (N. 165) are changed in a few seconds into octahedrons by the heat of the sun: other instances might be given of the influence of even moderate degrees of temperature on molecular attraction in the interior of substances. It must be observed that these experiments give entirely new views with regard to the constitution of solid bodies. We are led from the mobility of fluids to expect great changes in the relative positions of their molecules, which must be in perpetual motion even in the stillest water or calmest air; but we were not prepared to find motion to such an extent in the interior of solids.