REVIEWS 519 



extensively taught ever since, at least in America. The microscopic 

 examinations of the granules referred to were carried out by C. S. Peet 

 and E. C. Perisho daily throughout the rather severe winter of 1893-94 

 as experienced at Chicago. The granules and their whole immediate 

 environment were often much below the freezing point for days together. 

 Furthermore, in the paper cited, there was also a discussion of the 

 effects of the penetration of the snow fields by the "winter wave" of 

 the arctic region, and mean temperatures as low as — 25° C. were had 

 under consideration. But disregarding these explicit postulations of 

 granular growth at temperatures far below the freezing point, the writer 

 of this part of the report continues: 



The supposed mechanism seems to be a progressive melting under pressure 

 where the points of the crystals touch one another, and a flow of the fluid thus 

 produced to other places where the pressure is less. This might afford a reason- 

 able explanation of the cause of growth of the larger crystals at the expense of 

 the smaller in a mass of snow composed of individual crystals at temperatures 

 near the triple point, for the local pressure per unit area must be greater for the 

 smaller crystals, and therefore a greater amount of melting should take place 

 at their surface. It does not, however, afford any explanation of the fact that 

 crystals do slowly grow in size, even at temperatures well below zero Fahrenheit, 

 where the pressure due to the superincumbent foot or so of snow is utterly 

 insufficient to cause any significant local increase of pressure. 



The theory of yield and growth by pressure-melting at points of con- 

 tact has long been held for the range of temperatures to which it is appli- 

 cable but not urged, so far as we know, for any other. As growth at 

 lower temperatures of any degree had long been assigned to evaporation 

 and re-accretion, the last sentence has no pertinence except for the 

 author's unwarranted predication of the view "that the growth of certain 

 crystals at the expense of others takes place only [italics the reviewer's] 

 when the snow or ice is close to the melting temperature." 



The reviewer, at least, had never previously heard of a theory of 

 granular growth so restricted. On the contrary, the following from 

 Chamberlin's paper above cited has had much currency: 



To follow the process [of granular growth] it should be noted that the surface 

 of every granule is constantly throwing off particles of vapor, that the rate at 

 which the particles are thrown off is dependent, among other things, on the 

 curvature of the surface, being greater the sharper the curve ; that the surfaces 

 of the granules are at the same time liable to receive and retain molecules thrown 

 from other graniiles; and that, other things being equal, the retention of parti- 

 cles also depends on the curvature of the surface but in a reversed sense, the less 

 curved surface retaining more than the sharply curved one. Under these laws. 



