388 



NA TURE 



[February 27, 1896 



Institute on this subject is referred to. As the majority 

 of systems make use of heat suppUed by radiating sur- 

 faces, the author, in chapter iv., very fully explains what 

 is meant by the various terms, and then proceeds to 

 describe the methods of testing radiators at Sibley 

 College to measure the heat discharged. The results 

 obtained are brought together in a table ; further tests 

 being given from radiators with extended surface so as to 

 form air-flues. 



The remaining portion of the book deals with the 

 practical details and metallic parts of design, and archi- 

 tectural considerations. Much sensible advice is imparted 

 as to the care of steam-heating boilers, and how to avoid 

 boiler explosions, of which statistics are given. We notice 

 many useful rules and formulae for various purposes when 

 designing a hot-water or steam-heating system. The 

 volume lifts the subject out of the hands of the " plumber," 

 and leaves it in the hands of the " engineer." Prof. Car- 

 penter is to be congratulated on producing a really good 

 book on a subject seldom treated scientifically. 



Lessons in Elementary Botany for Secondary Schools. 

 By Thomas H. Macbride. Pp. 233. (Boston : Allyn 

 and Bacon, 1896.) 



In his far-reaching essay on "Education," Mr. Herbert 

 Spencer remarks : "In education the process of self- 

 development should be encouraged to the uttermost. 

 Children should be led to make their own investigations, 

 and to draw their own inferences. They should be 

 told as little as possible, and induced to discover as 

 much as possible." It is satisfactory to all who are 

 concerned in the progress of science, to know that these 

 sound principles of scientific instruction are being 

 brought more within the region of practical education 

 every day. The present volume is an addition to the 

 steadily growing literature in which the principles 

 referred to are applied. The young students, for whom 

 the book is intended, are led to make their own observa- 

 tions ; they are induced to study plants, rather than 

 printed books, and thus to derive their knowledge at 

 first hand from nature. The opening lesson in the book 

 is typical of the fifty-three which follow it. The pupils 

 are told to collect the twigs of various trees or shrubs 

 and to compare them, noting various peculiarities. A 

 single twig is then examined, and attention is directed 

 to the arrangement of the buds and leaf-scars upon it. 

 In the second lesson, twigs are compared with particular 

 reference to buds and their relations to branches, and 

 are grouped according to bud-arrangement. The structure 

 of stems afterwards forms the subject of several lessons, 

 and then the root, leaf, flower, fruit and seed are studied 

 in succession, after which come lessons having for their 

 object the elucidation of the structure and history of 

 individual plants and trees. Much more attention is 

 given to trees than is usual in books on botany. The 

 book is hardly suitable for class use on this side of the 

 Atlantic, but an English edition of it would be welcomed 

 by many teachers of botany. 



Vegetable Culture. By Alexander Dean, F.R.H.S. 



Pp. 136. (London : Macmillan and Co., 1896.) 

 Methods , and results are what amateur gardeners, 

 cottagers, and allotment-holders want, and this is the 

 book to supply their need in regard to the culture of 

 vegetables. Theirs not the ambition to ask the reason 

 why, but merely to know exactly what to do in order to 

 reap rich fruits of their industry. Very admirably does 

 the author impart this kind of information. In concise 

 language he describes the best methods to be followed 

 in the preparation and after-treatment of the soil, the 

 best varieties of the various classes of vegetables, and 

 the best systems of cultivation. Both the text and the 

 illustrations are instructive, and together they make up 

 a sound and serviceable primer for gardeners. 



NO. 1374, 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 NATURE. 

 No notice is taken of anonymous communications. '\ 

 The Rontgen Rays. 

 It may interest your readers to hear that, with the assistance 

 of Mr. J. C. M. Stanton, I have succeeded by means of the 

 Rontgen rays in actually seeing the coins inside a leather purse, 

 the metal instruments inside a closed wood and leather case, a 

 coin through a piece of wood half an inch in thickness, and also 

 through a sheet of aluminium. 



Photography was not employed, but the shadows of the 

 enclosed objects were made directly visible to the eye by means 

 of a fluorescent screen. 



The precise arrangement was similar to that recently described 

 by Prof. Salvioni, of Perugia, whose results, though in accord- 

 ance with certain experiments of Prof. Rontgen, confirmed, I 

 understand, by Mr. Porter, of University College, have so far 

 been received in this country with a certain amount ot 

 scepticism. 



The apparatus consisted of a tube of opaque paste-board with 

 a simple aperture at one end, to which the eye was applied. 

 The other end was provided with an opaque diaphragm of double 

 black paper, upon which, on the inner side, was laid a piece of 

 blotting-paper impregnated with platino-cyanide of barium in a 

 crystalline state. 



The purse or other object was held against the diaphragm with 

 the Crookes' tube beyond it, so that the rays from the latter cast 

 a shadow of the coins through the leather and black paper upon 

 the inner impregnated screen. The platino-cyanide fluoresced 

 brightly under the stimulus of the rays on those portions of the 

 blotting-paper where no shadow was cast, and consequently the 

 form of the metallic objects was made clearly visible. Non- 

 metallic objects were also clearly seen, though more faintly, 

 owing to their greater transparency to the rays. 



Besides being exceedingly interesting in itself, and possibly 

 capable of sufficient improvement to render it of service in 

 medicine and surgery, the appliance will be very useful for the 

 purpose of ascertaining, without the tedious process of exposing 

 and developing a plate, whether any given Crookes' tube is suit- 

 able as regards exhaustion and form for photographic purposes. 



It can be seen at once whether the tube is working to the best 

 advantage, and is giving clearly defined shadows. 



The place on the glass of the tube from which the maximum 

 radiation is proceeding, can also be easily determined, and I may 

 mention as confirming a point previously noticed by myself — i.e. 

 that a tube with a well-marked fatigue spot on the glass will not 

 answer satisfactorily for photography — that with the above- 

 described instrument the fatigue spot is visible to the eye through 

 the black paper, thus showing that the glass when fatigued does 

 not transmit the Rontgen rays. A. A. C. Swinton. 



66 Victoria-street, Westminster, February 25. 

 P. S.— Since writing the above, I have been able to see dis- 

 tinctly the bones in the thick portion of my own hand. 



As the hand appears to feel a cold sensation when exposed to 

 the X-rays, an experiment was made with the thermopile to put 

 the matter to the test. This showed that heat was being radiated 

 from the phosphorescent patch in the Crookes' tube ; if the 

 current be reversed so as to make the opposite pole the anode, 

 then heat was again radiated, but in a very much smaller 

 amount. The phosphorescent patch becomes very markedly 

 warmed in some tubes. On replacing the thermopile by a lighted 

 candle, the flame exhibited a flickering motion, and was slightly 

 drawn towards the phosphorescent patch ; this could be ob- 

 served at a distance of six inches. The phenomenon was also 

 observed when the candle was placed on the side of the tube 

 opposite the anode, but less markedly. A flame is almost trans- 

 parent to the X-rays ; on taking a shadowgraph of a lighted 

 candle or gas jet, the shadow of the flame is just visible as an 

 exceedingly faint impression, the internal core in the case of the 

 gas flame being slightly more marked than the external. 



Edinburgh. Dawson Turner, 



The Cause of an Ice Age. 



Though I have no wish to prolong this discussion, yet 

 will ask you to spare me space for a few lines, 



