170 SCIENCE PROGRESS 



tiated material on their upward path is not regarded as a probability. The 

 differences in density of underlying magmas are correlated, on the isostatic 

 principle, with the larger hollows and elevations of the earth's surface. 



The influence of radium in heating the earth's crust comes naturally under 

 discussion, and it is pointed out (p. 141) that there is not a proportionate rise of 

 temperature in the rocks of a district such as Joachimstal, where radioactive 

 minerals are abundant. We have touched on a few of the interesting problems 

 that must be faced, as Prof. Iddings shows, by the modern student of volcanoes. 

 The superficial manifestations, where molten silicates at temperatures of 1100 C. 

 appear among the outer rocks of the crust, point to the insecurity of the solid 

 lithosphere, and serve in part to explain its fracturing in the present and in 

 the past. 



G. A. J. Cole. 



An Amateur's Introduction to Crystallography (from morphological 

 observations). By Sir William Phipson Beale, Bart., K.C., M.P., 

 Treasurer of the Mineralogical Society. [Pp. viii + 220.] (London : 

 Longmans, Green & Co., 191 5. Price 4s. td. net.) 



It has often been remarked that the efficient scientific amateur is more common 

 in the British Isles than in countries where scientific knowledge is more generally 

 diffused. The explanation seems to be that in the latter countries men of scientific 

 tastes find a wide field of appreciative employers, and so drift into the professional 

 class. But the amateur, the man who works because he is fascinated by his 

 subject, plays a very great part in the mental uplifting of his State. He is less 

 likely to dogmatise, he is more open to new conceptions, than those who develop 

 their theses among the traditions of the schools. On the other hand, he is not 

 likely to be so resourceful in experimental methods as those who have ample 

 laboratories at command, and he usually turns to natural history rather than to 

 the physical sciences. 



Sir William Beale rightly regards mineralogy as a branch of natural history, 

 and he remarks that crystallography is to mineralogy what anatomy is to zoology. 

 He attracts the reader at the outset (p. 8) by developing crystallographic principles 

 from a fragmental specimen of a homogeneous substance, on which only a few 

 faces can be seen. He proceeds to refer these faces to a system of three axes 

 drawn respectively parallel to " any three edges between these faces not in one 

 plane and no two of the edges being parallel." The point of intersection of these 

 axes is shown to be quite independent of the centre of a symmetrical crystal. 

 The natural development of crystal-faces of the same form on very different scales 

 thus presents no initial difficulty. The crystal selected is triclinic, and forms 

 of greater symmetry are approached later in the volume. Those of us who were 

 brought up on other methods, and were led to regard the axes of reference as 

 something possessed by the crystal about which it was built up, will welcome 

 the author's natural and morphological treatment (p. 54). The insistence on the 

 derivation of the crystallographic axes from observed edges renders the use of 

 three equally inclined axes in the hexagonal and trigonal systems easily under- 

 stood. Many teachers must have found the immense convenience of starting 

 with forms consisting of two parallel faces only (p. 59), or even with the "pedions " 

 of P. Groth. The older mineralogists were attracted by the wonderful symmetry of 

 the cube and the forms derived from it by suitable truncation ; and their method 

 of approach has hampered students far into our own time. 



The possibility of the determination of the system of a crystal by symmetry, in 



