430 Sail and Snow. 



to have been curiously obstructed. The current which in a 

 clear solution sets constantly upwards from a forming crystal, 

 like the rejected stream from the cilia of a rotifer, bringing to 

 the crystal a steady supply of fresh particles for its aggran- 

 dizement, finds here an impediment to motion in the crowded 

 grains of foreign matter ; and tree-like, branching skeletons of 

 octahedra are the result of the crystallization, as if feelers had 

 been thrown out from a stem in quest of the saturated solution. 

 The obstructing action of the nitrogen and oxygen gases to 

 the crystallization of snow is of an exactly similar nature. 



In the case we have supposed, where vapour of extreme 

 tenuity is elevated, often by its own buoyancy, into regions far 

 colder than its dew-point, a process of direct sublimation takes 

 place. The crystallizing gas in this process bears no larger 

 proportion than two or three parts in a thousand to the obtru- 

 sive and far denser gases among which its snow crystals must 

 take their form. In the guarded operations of our laboratories 

 the sublimation of pure vapours is attended with the formation 

 of crystals of considerable solidity and often of great regularity, 

 but in the free play of the elements the hexagonal growth of 

 the ice crystals is developed into an endless variety of forms. 

 Messrs. Lowe and Glaisher have delineated some hundred ex- 

 amples of these figures. In place of the solid prisms peculiar 

 to the system of crystallization, these are flat, patterned, stars 

 of six rays, grotesquely ramified, and having diameters of a 

 twelfth or even a tenth of an inch. They are the skeleton - 

 bases of regular prisms, which, grouped together, form the 

 downy material of a snow-flake. 



Thus it is that conditions in the depth of winter are quite 

 unfitted for the formation of hail ; but in the microscopic rami- 

 fications of the snow crystals we also perceive a plausible reason 

 why hghtning and thunder should not in the winter season ac- 

 company falls of snow, as they so frequently do in summer 

 those of rain and hail. The large surface and the numerous 

 acute points which these crystals present, are, in all probability, 

 means sufficient for the complete and rapid dissipation of the 

 electricity which they accumulate by their assemblage. 



