NATURE 



[August 25, 1910 



nitrate), &c. To borrow the language of a sister 

 science, there exists a slight doubt as lo the veridical 

 nature of these phenomena. Howevei , an editorial 

 footnote commends them to the reader's notice. 



In an interesting article, H. Lunden gives a de- 

 scription (with sketch-plans) of Arrhenius' new Nobel 

 Institute laboratory at Stockholm. Several other short 

 articles deal with such varied topics as sun-spots and 

 magnetic storms, precautions against coal-dust ex- 

 plosions in mines, &c. Enough has been said _ to 

 indicate the catholicity of the editor's views concerning 

 the scope of his new journal. 



List of Documents in. Spanish Archives relating to 



the History of the United States, which have been 



printed or of which Transcripts are preserved in 



American Libraries. By J. A. Robertson. Pp. 



XV -I- 368. (Washington, D.C. : Carnegie Institution 



of Washington, 1910.) 



This publication of the Carnegie Institution is the 



most recent of the "papers" of the Department of 



Historical Research of the Institution at Washington. 



The editor of the series points out in a preface that 



the volume may be regarded as an accompaniment 



to Prof. W. R." Shepherd's "Guide to the Materials 



for the History of the United States in Spanish 



Archives." 



The two lists contained in the present book concern 

 the history of the territory included within the boun- 

 daries of the continental United States of to-day. 

 All matter touching that territory only indirectly or 

 by inference as a part of the Indies has been rejected. 

 The first list consists of published material, the 

 original manuscripts of which exist in Spanish archives, 

 or which, with good reason, are conjectured to exist 

 in Spain ; the second is much the longer, and is a 

 list of transcripts in libraries and archives in the 

 United States from originals in Spanish archives. 



Lightning and the Churches. By Alfred Hands. 



Second edition. Pp. 92. (London : J. W. Gray and 



Son, 1910.) Price is. net. 

 The first edition of this interesting pamphlet was 

 dealt with in a note in our issue of April 22, 1909 

 (vol. Ixxx., p. 228), and we welcome this second 

 edition as indicating that increased attention is being 

 directed to the important matter of protecting build- 

 ings from damage by lightning. Mr. Hands, who 

 has expert knowledge of the subject, says that inves- 

 tigation shows that about twenty churches are struck 

 and damaged in Great Britain every year. In some 

 years the number is much greater ; in 1907, for in- 

 stance, thirtv-nine suffered from this cause, and in 

 1908 there were thirty-one. Architects and others, 

 whose business makes them responsible for the pro- 

 tection of buildings against lightning, would do well 

 to study this little work. 



The British Isles in Picliires. A Geographical 

 Reading Book. By H. Clive Barnard. Pp. 

 64, containing 58 illustrations. (London : A. and 

 C. Black, 19 10.) Price is. 6d. 

 There are thirty-two beautiful coloured pictures in 

 this volume, which will delight young pupils of 

 geography, and serve also to explain graphically to 

 them the characteristics of many kinds of scenery 

 found in their native land. The black and white 

 illustrations will also help to secure and maintain the 

 interest of a class. The letterpress provides useful 

 information ; and, though it is hardly suitable for a 

 text-book, it will serve admirably to supplement the 

 geography lesson proper. The cheapness of the 

 volume should ensure it a wide popularity. 



NO. 2130, VOL. 84] 



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 0/ Nature. 

 No notice is taken of anonymous communications.] 



The Temperature Coefficients of the Ferromagnetic 



Metals. 

 Experiments on the temperature coefficients of magnets 

 published some years ago (Roy. Soc. Phil. Trans., vol, 

 cciii., 1903) showed that the magnitude ol the coefficient 

 is largely dependent on the dimension ratio of the magnet, 

 but that when the dimension ratio is sufficiently increased 

 to make the self-demagnetising factor negligible, the 

 coefficient is then characteristic of the ferromagnetic 

 material. Observations made at that time, but not pub- 

 lished, on iron, nickel, and cobalt magnets heated up to 

 100° C. showed that the coefficient was large in nickel, 

 less in iron, and least in cobalt, the order being the inverse 

 of the order of the magnetic critical temperatures of these 

 metals. 



1 have recently repeated these, experiments, carrying 

 the temperature to 3O0°-40o° C, and within this range 

 it appears that when the magnetic intensity in the cyclic 

 state is plotted against a scale, in which temperatures are 

 calculated as percentages of the critical temperatures, then 

 the points lie very nearly on one curve. In short, if 

 corresponding temperatures are chosen, the temperature 

 coefficient is the same for iron and nickel, and probably 

 for cobalt. (The details of the experiments I omit here, 

 but I should say that the cobalt employed was not pure, 

 and behaved irregularly at 300° C.) 



For ■ e.\ample, the temperature coefficient of nickel 

 between 15° and 115° C. is 0-0005, ^nd it is the same for 

 iron between 250'^ and 430° C, the corresponding tempera- 

 tures ; the temperature coefficient of iron between 7° and 

 107° C. is 00002, and it is nearly the same for cobalt 

 between go° and 220°, the corresponding temperatures. 



Curie has shown that when iron is heated it passes 

 continuously through the . critical temperature from the 

 ferromagnetic to the paramagnetic condition (in the latter 

 state obeying the gas laws mutatis mutandis), and that 

 the curves which trace the change from one state to the 

 other are the counterpart of the curves which trace the 

 passage of a liquid to a gas. An equation to the fluid 

 curves may therefore be applied to the magnetic curves, 

 and van der Waals's equation may be appropriately used, 

 =ince it takes account of the facts that there is mutual 

 attraction of the molecules and that there is a limit to 

 fluid density, facts which have their counterpart in ferro- 

 magnelism in the mutual attraction of the molecular 

 magnets and in the limit to magnetic intensity. One ot 

 the conclusions drawn from van der Waals's equation and 

 the theory of corresponding states is that the coefficient of 

 density, or expansion, of all liquids at corresponding 

 temperatures is the same, and by similar reasoning this 

 law is deducible for magnets. This is, in fact, the law 

 to which the experiments cited above lead. 



It is also worthy of notice that the temperature 

 coefficient of density of liquids is of the same order as 

 the temperature coefficient of intensity of ferromagnetics, 

 just as the temperature coefficient of density of gases is 

 of the same order as the temperature coefficient of intensity 

 of paramagnetics. 



Within the limits of this letter it is not possible to 



discuss these facts, but they show that certain magnetic 



problem's may be treated by an application of van der 



Waals's equation with results consistent with experiment. 



Manchester. J. R. Ashworth. 



The Ratio between Uranium and Radium in Minerals. 

 Dr. Boltwood established the constancy of this ratio 

 for all the minerals he examined (Phil. Mag., .^pril, 

 iqos). He examined, however, no mineral in which 

 uranium was present as phosphate, nor did he examine 

 the then newly discovered mineral thorianite. Later, 

 Mdlle. Gleditsch (Compfcs rendus, cxiviii., 145 1 ; cxiix., 

 207) found that the ratio radium to uranium was about 



