4S4 



NA TURE 



\J\Iarch 25, I < 



are principally considered, objects suitable for examina- 

 tion with larger ones up to seven inches are given. 



Much trouble has been taken with a very convenient 

 result ; and as the author shows what corrections to apply 

 to make the volume useful in future years, we must regard 

 it as one of the most useful books an amateur astronomer 

 can possess. 



Practical Introductioa to Chemistry. By W. A. Shen- 



stone. Lecture; on Chemistry in Clifton College. 



(London: Rivingtons, 1886.) 

 Although several courses of practical chemistry for 

 beginners have lately been introduced the author has not 

 found any of them suitable for school work, and so he 

 has undertaken to write one himself 



In his selection of experiments, he says he has been 

 guided by two main considerations. First, that they 

 should be suitable for the young boys who chiefly will 

 have to perform them, and who will have but a limited 

 amount of time to do them in. Secondly, that when 

 completed they shall constitute a body of experience 

 which shall be as valuable as possible to appeal to when 

 the students pass to the classes which have lecture 

 teaching. 



The first three chapters deal briefly with the elements, 

 compounds, acids, &c. Chapters IV. and V. deal with 

 the law of chemical combination and the classification of 

 chemical changes; Chapters VI. and VII. with the de- 

 composition of water and air. Chapter VIII. is devoted 

 to a few very elementary experiments on the relations 

 between solids, liquids, and gases. In Chapters IX. and 

 X. attention is drawn to the use made of these differ- 

 ences in experiment. 



The appendixes contain a list of apparatus and also a 

 description of the balance and how to use it. 



The book is divide 1 into two sections. In the first sec- 

 tion the student is given instructions how to perform the 

 experiments, but he is not told how they will " come out," 

 so that as the experiment proceeds, he has to observe and 

 note what takes place, and when it is finished he can 

 compare his notes with those given in Section II., where 

 full explanations are afforded. Certainly if this method 

 is well carried out we shall have a vast improvement 

 upon the ordinary "test tubing" process, in which, as a 

 rule, little theoretical construction is given to the be- 

 ginner. W. 



LETTERS TO THE EDITOR 



\7 he Editordoes not hold himself responsible/or opinionsexpress<il 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous comtnunications. 



yrhe Editor urgently requests correspondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that it is impossible othe)"wise to insure the appearance even 

 of communications containinginteresting and novel facts. ] 



Permanent Magnetic Polarity 



Since the subject of the permanent polarity of quartz has 

 been brought prominently forward by the researches of Dr. 

 Tumlirz, published in the January number of Wiedemann's 

 Annalcn, and by the recent discussion of it in your pages, it is 

 perhaps allowable for me to put on record the fact that I have 

 been engaged during the course of this winter with very similar 

 experiments, and have obtained very similar results. Qu.-irtz 

 indeed does not happen to be one of the substances I have 

 examined, — I rather dreaded the complexity of crystalline sub- 

 stances, — but my observations have led me to the conclusion 

 that most likely every substance possesses some trace of per- 

 manent m.ignetisability or retentivity. 



The set of experiments were not indeed begun with the object 

 of looking for permanent polarity, but with a wholly different 

 object, viz. this :— According to the Ampere-Weber theory of 

 magnetism and diamagnetism, wherein magnetism is explained 

 by means of specific molecular currents flowing in channels of 



no resistance, and diamagnetism by induced currents excited in 

 those same channels by the magnetic field, it is obvious that 

 '.he permeability of a magnetic body ought to become negative 

 when the magnetising force applied oversteps a certain amount. 

 Because an increasing magnetising force must weaken the specific 

 currents, even though it is unable to excite contrary ones and so 

 cause diamagnetism. 



This result of the theory is pointed out by Clerk-Maxwell 

 (vol. ii. § 844, 1st edit.), who further says : "If it should ever 

 be experimentally proved that the temjjorary magnetisation of 

 any substance first increases and then diminishes as the mag- 

 netising force is continually increased, the evidence of the exist- 

 ence of these molecular currents would, I think, be raised almost 

 to the rank of a demonstration." 



There are many circumstances now known which point more 

 or less distinctly to such a maximum, but my ambition has been 

 to not only establish a falling off of induced magnetism, but 

 actually to reverse it ; to convert, in fact, a feebly-magnetic 

 substance into a diamagnetic substance by immersing it in a 

 sufficiently intense m.agnetic field. 



Accordingly, in October last, I set up a fairly large magnet, with 

 .specially-pTiinted pole-pieces about a centimetre or less apart, 

 and arranged that various strengths of current, ranging from very 

 weak to very strong, might be sent round its coils ; the weakest 

 current being given by a Leclanche or two, an intermediate 

 strength by 3 or 4 secondary lead cells, a strong current hy 24 

 such cells, and the greatest strength by about 40 secondary 

 batteries, some of them zinc-lead with 2\ active volts apiece be- 

 tween their terminals, tept charged in two batches by a dynamo. 



I then instructed my workshop-assistant, Mr. Benjamin 

 Pavies, to fill up his odd time by cutting ellipsoids of all manner 

 of substances (axes about '6, '3, '3), to finish them off with 

 glass-paper, when practicable to boil them in acid, and then to 

 examine their behaviour between the poles of the magnet in a 

 specified way. 



The dimensions of the magnet were : — 



Diameter of iron core 5 centims. 



Length of each leg 21 ,, 



Distance from centre to centre ... 15 >. 

 Total number of turns of No. 12 B.W.G. wire, 



1 868, on both legs together. 

 Resistance of wire I'l ohm. 



Usual strengths of current, from \ ampere to 50 

 amperes. 



The thing intended was to discover by trial some substance so 

 feebly magnetic that, though it could just set itself axially with 

 the weakest current, it might lie equatorially with the strongest. 

 But failing this actual change of property it was thought that the 

 rate of oscillation between the poles might diminish for some 

 (non-conducting) substances when the highest powers were 

 applied, instead of increasing. 



And meanwhile the behaviour of all the substances was to be 

 noted and carefully recorded, whatever it might be. 



In this way a large number of substances, various kinds of 

 wood, all sorts of metal, glass, coke, charcoal, wax, chall<, card- 

 board, ebonite, &c., have been passed under review ; and some 

 one or two of them seemed to behave exactly in the way hoped 

 for. One piece of coke, for instance, vibrated in the intense 

 field more slowly than it did in the feeble one ; while another, 

 which vibrated axially in a weak field, set itself nearly equa- 

 torially in the strong one. Its behaviour was thus sufficiently 

 like what we wanted to justify a more careful examination. 



Soon after this, however, Uavies of his own accord inserted a 

 reversing key into the circuit of the Leclanche, and thus made 

 an important observation. 



When the strong current was reversed, the deportment of the 

 substance remained unaltered, as is natural enough ; but directly 

 the weak current was reversed, the little suspended piece turned 

 in the magnetic field through 120° or so, and pointed in a 

 symmetrically situate direction on the other side the magnetic 

 axis. The piece of coke, for instance, which may have been 

 pointing some 60° on the one side of the magnetic axis, changed 

 its position when the magnet was reversed, and pointed some 60" 

 on the other side. The suspending thread was not wholly 

 devoid of torsion though it was extremely minute. A piece of 

 electrolytic copper, and a piece of boxwood with the grain long- 

 ways, wei-e soon afterwards found, which set themselves almost 

 exactly equatorially, and on reversing the magnet turned through 

 very nearly 180°. 



I was a little excited about this result at first, because I thought 



