June 23, 1898] 



NATURE 



^12> 



The first part contains a classification of all active 

 substances known, and a succinct account of the theory 

 of \'an 't Hofif and Le Bel. The properties of the active, 

 racemic and inactive modifications of a substance are 

 then contrasted, and the methods of converting them 

 into and separating them from each other described. A 

 chapter by Prof. Lindner, on the micro-organisms em- 

 ployed in splitting up racemic compounds into their 

 constituents, should be helpful to chemists. In the third 

 part the rotation is considered from the physical point 

 of view, the chapter on the influence of solvents on 

 rotation being especially interesting. Many of the phe- 

 nomena observed are still unexplained, and it would 

 appear that a study of these should be capable of 

 throwing some light on the nature of solutions. After a 

 discussion of Guye's hypothesis, which is found to be 

 insufficient, the author remarks that it will probably be 

 impossible ever to discover the numerical connection 

 between chemical constitution and rotation. 



One hundred and forty-two pages are devoted to a 

 very excellent account, by Dr. O. Schonrock, of polari- 

 meters and saccharimeters, the subsidiary apparatus 

 connected with them, and the methods of using them. 

 Dr. Schiitt contributes Part 5, on saccharimetry and 

 the determination of several other active substances of 

 technical importance, and the book terminates with a 

 collection of the rotatory powers of all active substances 

 known, which is complete up to the middle of 1896, and 

 includes some of the data published since that date. 

 A good index is added. T. E. 



OUR BOOK SHELF. 

 The Span of Gestation and the Cause of Birth. By John 



Beard. Pp ix -I- 132. (Jena : Gustav Fischer, 1897.) 

 Commencing with the assumption that there is a 

 " critical period " in the development of every mammal 

 " when the embryo is first beginning to look like the form 

 whose offspring it is," Dr. Beard proceeds, in this mono- 

 graph, to point out the close connection existing between 

 the extent of time, or " critical unit," which elapses before 

 the " critical period " is attained and the ovulation and 

 total gestation periods. 



Dealing shortly with the probability of an alternation 

 of generations in mammals, which he has so ably advo- 

 cated in earlier communications, he reaffirms now his 

 previous conclusion that the attainment of the " critical 

 period " is coincident with the completion of all the 

 important parts of the sexual generation, and with the 

 commencing degeneration of the asexual generation or 

 phorozoon. The length of the " critical unit " is, there- 

 fore, the length of the life of the phorozoon, and when it 

 is completed, in the more primitive forms, c.(^. the mar- 

 supials, the birth of the sexual generation occurs. 



Obviously the simpler conditions prevailing in the 

 lower forms have been altered in the higher mammals, 

 and at first sight the alterations have not occurred along 

 definite lines, for the "critical unit" is not a fixed quantity ; 

 on the contrary, it varies in length from 7:^ days, in the 

 opossum, to 47 days, in man. Dr. Beard is convinced, 

 however, that the variations can only occur in conformity 

 with some discoverable law, and he shows that the 

 " critical unit " is either slightly less than one, or than 

 two combined ovular periods, which he proposes to term 

 "ovular units." He suggests that if ovulation was not 

 previously restricted it became impossible when gestation 

 was astablished, and could only recur, in the most favour- 

 able circumstances, shortly after birth, and thus the 

 NO. 1495, VOL. 58] 



" critical unit " came to govern the " ovulation unit." But 

 the intimate correlation between the critical and ovulation 

 units is not closer than that which exists between the 

 " critical unit" and the gestation period, for the latter is 

 always some multiple of the former, and the greater the 

 number of the "critical units" contained in the gestation 

 period the greater is the stage of the development of the 

 foetus at birth ; nevertheless, the completeness of the de- 

 velopment of a foetus at birth is not dependent merely 

 upon the length of its gestation period, but upon the 

 number of critical units in that period, for the " critical 

 unit " has probably been doubled or trebled in certain 

 cases, and the author believes that such lengthening is 

 associated not with increase of the development, but 

 only with increase in the size of the foetus. 



The points raised in this interesting memoir are clearly 

 stated, the evidence in their support is well arranged, and 

 the author is to be congratulated on having thrown light 

 on some obscure problems. It is to be hoped that he 

 will push his observations further, and that he will 

 eventually succeed in demonstrating " the cause of birth." 

 Arthur Robinson. 



A New Astronomy. By Prof. David P. Todd, M.A. 



Ph.D. Pp. 480. (New York, Cincinnati, Chicago : 



American Book Company.) 

 Astronomy is pre-eminently a practical science, yet in- 

 struction in it, and especially in the branch which per- 

 tains to geography, usually consists of a course of study 

 of text-books. This is not as it should be. It is far 

 better to observe the apparent movements of the stars and 

 planets than to learn that they are hundreds of thousands 

 of miles away from us ; and to note the annual movement 

 of the sun among the stars is more instructive than to 

 learn the dimensions of some sun-spots and prominences. 

 In astronomy, as in other sciences, the only firm con- 

 ceptions are those obtained from direct observation. 

 Prof. Todd's book marks a new departure by showing 

 how the fundamental principles of the subject may be 

 studied with the aid of tangible objects, somewhat as in 

 physics and chemistry. The resuit is most successful. 

 No book with which we are familiar contains a clearer 

 account of astronomical geography, and certainly none 

 show so well how to observe celestial movements or 

 illustrate astronomical phenomena with simple appli- 

 ances. The pupil who learns astronomy through Prof. 

 Todd's book will have a real idea of the motions and 

 measurements of the heavenly bodies instead of abstract 

 conceptions concerning them. 



The practical presentation of what may be termed 

 the geometry of astronomy only forms, however, one 

 commendable feature of the book. Other characteristics 

 which call for just as much praise are the large number of 

 illustrations — well reproduced and well chosen — and the 

 attention that is given to the advances made in recent 

 years in all branches of celestial science. Throughout the 

 book the endeavour has been to present the subject in a 

 way which will induce the student to think for himself, 

 and not merely commit facts to memory. In other words, 

 Prof. Todd shows how astronomy may be given an edu- 

 cational value, instead of being presented as a collection 

 of isolated and imperfectly connected facts. Fortunate is 

 the pupil whose teacher instructs him in astronomy on 

 the sound methods described in this book. 



Lessons in Domestic Science. Part i. By Ethel R. Lush. 



Pp. viii + 88. (London: Macmillan and Co., Ltd., 



1898.) 

 This instructive little book has been prepared for use 

 by children in public elementary schools. It contains 

 simple information on food, clothing, and personal hygiene, 

 and is well adapted for the purpose for which it is in- 

 tended. Wherever possible, the principles described are 

 illustrated by experiment. 



