312 



NATURE 



[November 20, 1919 



Judged from the book alone, it is perfectly 

 clear that Dr. Wood is well qualified to write 

 upon the subject he has taken up. Although some 

 of the subjects are dealt with sketchily enough, 

 he gives the impression that he has had experi- 

 ence ; that he has worked in the field ; that he has 

 kept his eyes open and knows generally what 

 should be done and how to do it. The American 

 health officer and student of public health topics 

 may very safelv take him as a guide. The English 

 health worker even may find something of value 

 in what he has to say, though he has no refer- 

 ence to English works and workers, and his view- 

 point is purely American, and his methods, most 

 of them, not such as are or could be applied in 

 this country. 



The book is not a large one, but Dr. Wood 

 covers the greater part of the field of health work, 

 dealing with such subjects as statistics ; control 

 of communicable diseases ; child welfare ; school 

 hygiene ; pure foods, etc. 



The treatment throughout is practical, and the 

 writing is good and attractive. The same may 

 be said of the illustrations, of which there is a 

 fair number. As already hinted, it is unlikely that 

 Dr. Wood's book will have more than a limited 

 appeal in this country, but it will probably receive 

 a good welcome from and be found useful by 

 health workers in the United States. 



The Study of the Weather. By E. H. Chapman. 



(The Cambridge Nature Study Series.) Pp. 



xii-l-131. (Cambridge: At the University 



Press, 1919.) Price 3s. 6d. net. 

 This little book on elementary meteorology will 

 be welcomed by the school-teacher, to whom it 

 makes its primary appeal. Though the serious 

 student of the science may at first feel that it 

 has no place on his shelves, yet, should he at 

 some time be called upon to lecture to a non- 

 scientific audience, he will find a perusal of its 

 pages of no small value. The matter dealt with 

 is mainly confined to features of the weather 

 which can readily be observed by young people 

 without special apparatus, and it is presumably 

 for this reason that any reference to pressure and 

 temperature conditions in the upper air is omitted. 

 With the foregoing rather notable exception the 

 groundwork of meteorology is well covered. One 

 of the chief features of the book is the series of 

 exercises, of which more than 250 are given. 

 These vary from some very simple questions to 

 others which the teacher would be well advised 

 to think out carefully before putting to his class 

 if he wishes to avoid finding himself in an awk- 

 ward position. Many of these questions are 

 calculated to arouse a most healthy interest in the 

 minds of the pupils. One example must sufifice. 

 "What kind of weather is it that causes the inside 

 walls of a building to stream with moisture? " 



The Cambridge University Press is to be con- 

 gratulated on the clearness of the printing and 

 the excellence of the get-up of the book. The 

 frontispiece is particularly pleasing. Numerous 

 illustrations and charts are included in the text. 



J. S. D. 

 NO. 2612, VOL. 104] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications. 'i 



Percussion Figures in Isotropic Solids. 



In the issue of Nature for October 9, Prof. C. V. 

 Raman, of Calcutta, illustrated the conical fracture 

 produced by the impact of a steel ball on a plate glass 

 surface. 



The following observations, which may be regarded 

 as supplementary, were made by the writer some time 

 ago with the object of finding what really happens 

 when a glass surface is being ground, or, as it is 

 technically termed, smoothed by an abrasive such as 

 carborundum. Individual grains of a good abrasive 

 have a nodular form, and the abrasion of glass appears 

 to arise from the impact or pressure of the grains. 



Two polished surfaces of glass were placed face to 

 face with a few grains of carborundum between therm, 

 and the specimen was compared with a similar one in 

 which steel balls of i mm. diameter were substitutiid 

 for the carborundum. Pressure was applied uni- 

 formly over the whole surface, and while the 

 pressure was being applied, the plates could be trans- 

 lated one over the other, thus producing the actual 

 machine conditions. The observations were made by 

 means of a polariscope. .As the appearances were 

 identical, steel balls were used throughout the later 

 experiments, thus enabling the conditions to be better 

 controlled. 



It will be assumed that the polished appearance of 

 glass is due to an amorphous surface layer. When 

 the surface particles are acted upon by mechanical 

 forces, the molecules, or possibly groups of molecules, 

 rearrange themselves, the result being akin to the sur- 

 face of a liquid. This conception was first advanced 

 by Lord Rayleigh, and there is now a large mass of 

 supporting evidence. When a piece of glass is worked 

 mechanically, the surface molecules are so profoundly 

 agitated that they are able to rearrange themselves 

 under the action of intermolecular forces. 



Fire glazing similarly consists in thermally agitating 

 the molecules. Very small forces are sufficient to 

 weaken the molecular cohesion by the required amount. 

 Chemical action may produce a similar result. An 

 optical surface may be reduced quite uniformly by the 

 action of HF, provided the fluorides as formed 

 are not allowed to crystallise and the bath is kept 

 in continuous movement. When a piece of glass is 

 fractured comparatively slowly, the forces at the edge 

 of propagation of the fine crack must be very great, 

 and, as before, the molecules are able to flow or 

 rearrange themselves to form a polished surface layer. 

 But when the fracture takes place suddenly, and 

 almost explosively, as, for example, in the cooling of 

 a fwt of optical glass, portions of the surface may 

 have a matt appearance to the unaided eye. This 

 type of matt surface has been discussed very fully by 

 M. Charles de Freminville, of Paris, who regards it 

 as a type of multiple fracture. This explanation is 

 more probable than the alternative one that the time 

 of fracture is too small to permit of viscous flow. 



When a steel ball is pressed lightly on the polished 

 surface of a glass block, the appearance, when viewed 

 between crossed Nicols, is as in the diagram (Fig. i)» 

 The central black cone has an angle of about 

 20°, which remains practically independent of the 

 pressure of the ball. The cone of strain b, b has 



