436 



NATURE 



[March 12, 1896 



OUR BOOK SHELF. 



Experi7nc7ttal Plant Physiology. By D. T. MacDougal. 



(New York : Henry Holt and Co., 1895.) 

 This little book is intended primarily for the use of 

 elementary students of vegetable physiology, and the 

 subjett-matter is treated in a somewhat dogmatic fashion. 

 This we cannot avoid regarding as a mistake. The 

 elementary, no less than the more advanced, student 

 requires to be made at the outset to think for himself^ 

 and this is a far more important matter than the mere 

 following out of directions for experiments. But it is just 

 this stimulus that we miss, in the pages before us, and in 

 not a few instances the author has sacrificed reality at 

 the shrine of conciseness ; thus it is simply useless to 

 compare the mechanism by which water " is taken up 

 and forced into the root and upward in the stem " to the 

 action of an osmometer, for the conditions are not at all 

 the same. The state of the water, as it exists in, and finds 

 its way upward into, the tracheids, is by no means 

 directly comparable with that of the solution of salts as 

 it occurs in the tube of a thistle funnel, the lower end of 

 which is closed by a piece of membrane. Here was a 

 good opportunity of insisting on the peculiarity of a living 

 protoplasmic structure, and it was lost. Very few 

 teachers, who are at all ate couratit with their subject, 

 will accept the statements as to ascent of sap, given on 

 pages 28 and 29. " The rectangular wood cells are in 

 the form of a series of chains. Water transpired from 

 the topmost cells of these chains, the cell sap becomes 

 concentrated. . . . There is thus formed a series of 

 osmometers extending from the leaves to the roots and 

 capable of lifting water to any height." Surely a slight 

 knowledge of elementary physics, combined with that of 

 the structure of the " wood cells," should have rendered it 

 impossible that such a passage could have been written. 



Again on page 46, one is tempted to wonder whether 

 the author can have ever tried the experiment (54) to 

 prove that oxygen is necessary for respiration. Any 

 unwary teacher who perforined that experiment for the 

 first time before an intelligent class, would probably 

 have a hard time of it if he trusted himself to Mr. 

 MacDougal's guidance. 



The above remarks and quotations will show that there 

 are weak points about the book. And, indeed, the general 

 impression left on the reader's mind is the rather painful 

 one, that an important branch of botanical study has here 

 been but superficially handled. 



Exercises in Physical Measurement. By Dr. L. W. 

 Austin and Dr. C. B. Thwing. Pp. x + 193. (Boston : 

 Allyn and Bacon, 1896.) 



After working through the experiments described in 

 this volume, a student will have a thorough knowledge 

 of the instruments and methods employed in physical 

 measurement, and will be in a position to accurately 

 investigate physical phenomena. The book is a medium 

 between such a hand-book as the last edition of 

 Kohlrausch's " Praktische Physik" and the elementary 

 text-books in which mathematical formulae are eschewed. 

 The sixty-five exercises in the first part are all quantita- 

 tive, and they cover measurements in the chief branches 

 of physics, beginning with measures of length, and 

 ending with experiments on polarisation ; while the 

 apparatus required for the experiments should form 

 part of the equipment of all physical laboratories in 

 which students can obtain scientific instruction. 



In the second part of the book suggestions are gi^iven 

 as to methods of computation and physical manipulations, 

 while the third part is taken up with tables. The book 

 is thoroughly practical and trustworthy, and it can be 

 recommended as a suitable introduction to serious work 

 in physics. 



NO. 1376, VOL. 53] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions ex- 

 pressed 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 Naturk. 

 No notice is taken of anouyinons coniiiiiini(ations.'\ 



The Age of the Wealden. 

 The Wealden formation of England has long been studied, 

 and is now well known in nearly all its features. Its strati- 

 graphical relations and its Cretaceous age are usually regarded 

 as fully determined, and this is true, also, of the corresponding 

 strata on the continent. 



The vertebrate fauna cf the Wealden is of special interest, 

 and has attracted much attention ever since Mantell in 1825 

 unearthed Iguanodon in the Tilgate Forest. Some of the 

 remains, he sent, through his friend Prof. Silliman, to Yale 

 College, where they have proved of much service. In 1865, I 

 examined the same famous locality, as well as others on the 

 south coast of England especially rich in vertebrate fossils, and 

 at all of them secured interesting specimens. A study of these 

 in connection with the collections at London and Brussels first 

 caused me to question the Cretaceous age of the Wealden, and 

 a later comparison of its reptilian fauna with allied forms found 

 in the Rocky Mountains led me to the conclusion that both 

 series were Jurassic in type, and should be placed in that 

 division of the geological series. 



At the meeting of the British Association at Ipswich, in 

 September last, I read a paper on European Dinosaurs, 

 including two from the Wealden, and thus the question of their 

 geological age came up for determination. The facts I then 

 presented, based mainly upon the reptilian fauna, strongly 

 indicated the Jurassic age of the Wealden, and I urged a re- 

 examination of the question by English geologists. The subject 

 has since been taken up by Smith Woodward, with special 

 reference to the fossil fishes. In the Geological Magazine for 

 February 1896, he gives the main results of his investigation, 

 which prove that the fishes, also, of the Wealden are of 

 Jurassic types, thus placing the geological age of this formation 

 beyond reasonable doubt. The concluding statement of this 

 interesting article is as follows : 



' ' The Wealden estuary seems to have been the last refuge of 

 the Jurassic marine fish-fauna in this part of the world, not 

 invaded even by stragglers from the dominant race of higher 

 fishes which characterised all the seas of the Cretaceous period. 

 The Wealden river drained a land where a typically Jurassic 

 flora flourished ; the only two known Mammalian teeth from the 

 Wealden resemble those of a Purbeckian genus ; and now it is 

 clear that the fishes agree both with these and the reptiles in their 

 alliance with the life of the Jurassic era." O. C. Marsh. 



Vale University, New Haven, Conn., February 22. 



The Rontgen Rays. 



The following fact regarding the X-rays of Rontgen may be 

 of interest. 



I have found that it is possible to obtain a photographic 

 image by these rays, using a " pinhole camera," having the 

 aperture pierced in apiece of sheet-lead backed with aluminium. 

 The Crookes' tube was illuminated by discharges from a 

 Thomson high-frequency coil. The photographs taken in this 

 way show very distinctly the two electrodes, while the glass 

 bulb, which appeared to be brightly illuminated to the eye, is 

 scarcely perceptible It would appear from this that nearly, if 

 not all, the so-called X-rays proceed directly from the electrodes 

 of the tube, and not from the glass where this is acted on by the 

 kathode rays. It likewise affords further illustration of the 

 rectilinear motion of the X-rays. Experiments are in progress 

 with a broken current, and also to study the effect of a magnetic 

 field. 



Previous observation has shown that photographic effects were 

 produced equally whether the kathode rays impinged upon the 

 glass, or upon other phosphorescent material {e.g. aragonite) 

 within the tube. It has also been noticed in experiments in this 

 laboratory, that the appearance of the tube to the eye affords no 

 criterion of its efficiency in producing the X-rays ; tubes show- 

 ing but little fluorescence of the glass composing them often 

 giving admirable photographic effects, which in some cases are 

 obtainable even from a low-vacuum Geissler tube. But the rays 



