﻿ATOMIC HEAT TO CRYSTALLINE FORM. 125 



in favor of the opaque variety — another departure from the usual rule, the opaque 

 form, having commonly less than the transparent or vitreous. 



The limits to which I have assigned this paper, prevent me from here introducing 

 in detail many facts similar to the above. I would, however, especially refer the 

 reader to the first volume of Gmelin's Chemical Hand-Book, where, in the article on 

 Amorphism, have been collected from the writings of Rose, Fuchs, Regnault, Frank- 

 enheim, Marchand, Scherer and others, many valuable and instructive examples of 

 the modifying influence of caloric upon the state of aggregation, form and properties 

 of bodies. These constitute but a few of the many examples which a progressive 

 science is daily placing upon record, to be used hereafter for the loftiest purposes of 

 philosophical generalization. They are worthy of, and will amply repay, the closest 

 investigation, since they conduct us to the very threshold of Nature's secret laboratory, 

 promising to lay bare that for which the human mind, through all ages, has so fondly 

 and unceasingly longed — a knowledge of the primary physical cause of all natural 

 phenomena. 



Nature annually performs for us, on a grand scale, an experiment at once simple, 

 beautiful and instructive. I allude to the formation of snow crystals. In this process 

 the crystallizing material is the same throughout all the varieties of form, but the 

 accompanying temperature varies. In his excellent work on the Arctic Regions, 

 Scoresby has classified the many forms of snow-flakes observed by him in his voyages 

 to the North, and given a table showing the different temperatures at which different 

 crystals were seen to fall. He refers the varieties of snow crystals, for their cause, 

 to variations in atmospheric temperature. My own observations made during the 

 past winter, lead to the same conclusion. When, after variable weather in the winter 

 season, the cold has set in steadily for several days, gradually condensing the aqueous 

 vapor and obscuring the heavens, the wind constantly blowing from the same point, 

 and the upper atmospheric currents, as shown by the course of the clouds, having the 

 same direction as the lower, I have very generally found that while the thermometer 

 indicates nearly the same reading, the crystals of snow that fall under such circum- 

 stances have the same or but slightly differing forms. Any marked change in the 

 height of the mercurial column is very commonly followed by some manifest change 

 in the snow-figures. It is true that very different and unrelated forms are often 

 observed to fall simultaneously ; but reasons well known to the climatologist, render 

 it very probable that they have been generated in atmospheric currents or strata 

 differing in temperature. Indeed I suspect that a snow-crystal, by passing through 

 variously heated currents of air, may have its form considerably altered as it descends 

 towards the earth. By holding the partly-closed hand over a crystal, so as to increase 

 the warmth of the surrounding air, I have, on several occasions, noticed very remark- 

 able modifications of form. Mr. James Glaisher also speaks of this phenomenon, in 



