﻿12G MEIGS ON THE RELATION OF 



a communication directed to the Editor of the Illustrated London News, (Feb. 24th, 

 1855.) He says, "In conclusion, I may be forgiven if I remark, for the benefit of 

 the inexperienced, that the crystal subjected to the warm influence of the proximity 

 of the observer, not unfrequently alters in form before it sensibly begins to melt. I 

 have myself more than once drawn a specimen on its first descent, verified it to my 

 satisfaction, and passed on to others ; after the lapse of perhaps a quarter of an hour 

 or more, I have been surprised by witnessing, as I imagined, a new specimen occu- 

 pying the precise spot of the former. In each case, however, careful observation 

 proved to me that it was no other than the crystal I had already drawn, and that the 

 dissimilarity arose from a general simplification of the figure caused by the subsidence 

 of the groups of prisms surmounting the apex of the rays, the rounding of the edges 

 of those prisms which remained, the subsiding of two or more prisms into one, and 

 the elongation of spiculae which formerly served as the axes of prisms." 



Very peculiar changes may be witnessed by studying the gradual wasting of a snow 

 crystal in consequence of an increasing warmth of the air. Thus I have seen a 

 hexagonal lamella similar to fig. 87 of Scoresby's plates, change into one like fig. 28, 

 then resemble a star, (fig. 13,) and ultimately leave only its three axes as the skeleton 

 or fundamental portion upon which the body of the crystal was erected. These axes 

 finally disappeared, the destruction beginning at their apices and ending at their 

 common centre or junction, thus following an order the reverse of their formation. 

 An exact statement of the amount of heat combined with each variety of snow-crystal 

 would constitute a valuable addition to our present stock of knowledge. 



Thus, then, we learn that the particular system or genus in which a body crystal- 

 lizes, is mainly determined by the temperature at which solidification takes place. 



Dimorphism is intimately associated with isomerism. Both phenomena furnish us 

 with instances of the positive and active relation which atomic heat bears to both the 

 form and properties of bodies. Some isomeric substances, though agreeing in ultimate 

 ponderable compositions, differ completely in atomic weight, and every other respect ; 

 in others, both the atomic weight and elementary compositions are the same ; in others 

 the atomic weights are the same and their molecular arrangement different; but, in 

 all, as far as I can ascertain, their atomic heats are different. Phosphoric and pyro- 

 phosphoric acids are alike in composition but dissimilar in properties : the second acid 

 is simply the first in a state of fusion. Racemic and tartaric acids are very similar in 

 properties and chemical relations, and, according to Mitscherlich, isomorphous. When 

 heated, they each form three classes of salts, but are so modified that each, upon unit- 

 ing with the same base, in the same proportion, produces a different compound. When 

 destructively distilled, both give rise to the same pyr-acid. It is well known, that 

 every organic acid has its corresponding pyr or fire acid. 



" When the group of chemical molecules are differently arranged," says Kane, " the 



