February 2, 1893] 



NATURE 



315 



dumps of protoplasm, removed from either a unicellular 

 plant or animal, in which no nucleus is present, although 

 capable of living, and retaining their irritability and 

 power of movement for some time, yet neither grow, nor 

 form a cell membrane, nor have the same power of 

 •digesting bodies introduced into their substance, as is 

 possessed by a clump of protoplasm which has retained 

 the nucleus. The nutritive activity of the protoplasm 

 would appear, therefore, to be under the influence of the 

 nucleus. 



The volume concludes with a chapter on the cell in its 

 relation to theories of heredity. The author, as is now 

 the prevailing opinion amongst biologists, contends that 

 the nucleus is the conveyer of hereditary properties, and 

 that the offspring is a mixed product of both its parents, 

 •derived from the ovum and the sperm cell. In the course 

 of this chapter he discusses the views of Darwin, Spencer, 

 Nageli, Weismann, and De Vries, and suggests the em- 

 ployment of the term "Idioblasts" for the minute 

 €lementary particles, which Darwin called " gemmules" 

 in his hypothesis of pangenesis, and which he conceived 

 to be capable of transmitting hereditary characters to 

 succeeding generations. 



THEORETICAL MECHANICS. 



Elementary Mechanics of Solids and Fluids. By A. L. 

 Selby, M.A. (Oxford: Clarendon Press, 1893.) 



AT a period when we are bound to recognize the 

 influence exerted by the examinations of the 

 various educational institutions and of those controlled 

 by other more or less influential examining bodies, we 

 may be excused, on the arrival of a new work, for stating 

 whether or not, and to what extent, it is adapted to their 

 requirements. The book before us does not appear to 

 have been intentionally written for examination purposes, 

 and perhaps on this account it will be all the more wel- 

 come. Its purpose, however, is very distinct. It is 

 intended for those students who are desirous of reading 

 mechanics as an introduction to a study of physics. So 

 far, therefore, as its suitability for examinations is con- 

 cerned, we can heartily recommend it to those who wish 

 to qualify in this particular branch of science, while at 

 the same time it will be read with great benefit by that 

 class of students who desire a thorough knowledge of the 

 portions generally included under the head of Theoretical 

 Mechanics. 



In the study of such subjects as the book treats of, the 

 amount of knowledge which the reader may have of 

 mathematics will, to a considerable extent, be a measure 

 of his success. The author expresses a hope that an 

 acquaintance with the elements of algebra and geometry 

 will suffice ; but, while not wishing to reduce the useful- 

 ness of the book, but rather to direct it into proper hands 

 in which it will be read with greater advantage, we think 

 it would be nearer the mark to say that a thorough know- 

 ledge of elementary algebra and a considerable acquaint- 

 ance with elementary trigonometry are necessary. 

 Certainly the definitions of the trigonometrical ratios will 

 be found in an appendix, but it will be far better if the 

 student has lived with and used these for some time. 



Possessing these requirements, he will appreciate 

 and even admire the broad, yet concise nature 

 NO. 12 14, VOL. 47] 



of the treatment generally ; and with regard to this 

 matter we may say that we are unacquainted with any 

 elementary text-book better calculated to create a desire 

 for precise and full ideas. That this is requisite for a 

 study of physics perhaps more than any other subject, no 

 one will deny. 



The first chapter of the book is occupied with a con- 

 sideration of Kinematics, and in it will be found a careful 

 exposition of the displacement, velocity, and acceleration 

 to which a body may be subjected, due attention being 

 drawn to what is necessary for a full representation of 

 them. The appendix following this contains some 

 geometrical theorems and definitions for subsequent use. 

 Then follow the usual chapters on the laws of motion, 

 work, and energy, centre of gravity, moments of inertia, 

 and simple machines. A chapter on gravitation will be read 

 with interest, preceded as it is by an explanation of some of 

 the geometrical properties of the ellipse. Kepler's laws of 

 planetary motion are dealt with, in addition to other 

 relevant matters which do not usually find their way into 

 elementary text-books. 



The subject of elasticity also receives a somewhat more 

 extensive treatment than is usually given to it. The various 

 kinds of stress and strain which a body may undergo are 

 explained, together with the relation between stress and 

 strain. At the end of the book we find what is included 

 under the second head of the title. The various principles 

 and laws which refer to fluids, and some of the machines 

 and instruments which depend on them for their action, 

 are enumerated and explained, while the interesting sub- 

 ject of capillarity has a separate chapter devoted to it. 



To an appreciative reader it is a source of satisfaction 

 to observe the care the author has exercised when deal- 

 ing with the important matter of definitions and units- 

 fundamental and derived. A chapter on units and their 

 dimensions is furnished at the end. 



A good selection of examples, bearing on the matter 

 treated therein, will be found at the close of the 

 chapters. 



Many portions of the book are characterized by a 

 decided freshness of treatment, and we have little doubt 

 that the careful reader will find many little points which 

 are satisfying, in that they tend to widen the somewhat 

 restricted views he may have previously held, and these 

 will be all the more apparent should his mind be of a 

 mathematical turn. G. A. B. 



OUR BOOK SHELF. 

 Magnetism and Electricity. By R. W. Stewart. (London: 



W. B. Clive and Co.) 

 The book forms one of the University Correspondence 

 College Tutorial Series, and is "primarily written for 

 the use of candidates for the Matriculation, Intermediate 

 Science, and Preliminary Scientific Examinations of the 

 University of London." The author is evidently familiar 

 with the difficulties which usually occur to students, and 

 the best portions of the book are those in which efforts 

 are made to elucidate some of the more general errors. 

 The descriptions of apparatus and phenomena are, how- 

 ever, generally rather short and meagre, while the dia- 

 grams are frequently inadequate for a work of this sort. 

 Little is written to help the beginner to perform experi- 

 ments for himself ; in fact, descriptions of many important 

 instruments are omitted— for example, the Wheatstone 



