May II, 1888.] 



SCIENTIFIC NE^VS. 



447 



evening under competent direction will soon get a 

 likely student past his elementary difficulties. 



If no more is done than this, the result may be far 

 from contemptible. We have known a village club, in 

 which the directing spirit was the classical master of a 

 neighbouring grammar school, who had himself taken 

 out an elementary biological course at the University. 

 He gathered youngfellowsabout him, and they dissected, 

 mounted microscopic objects, rambled out together, and 

 made collections. It would be hard to name a field-cIub 

 •which was more thorough-going, or more enthusiastic. 

 Unfortunate!}', the fortunes of the enterprise turned 

 upon one man only, and when he went all was over. 



As soon as elementary methods have been mastered, 

 a wide field is open to the amateur student. We will 

 name a few of these, in which real skill is attainable with 

 a moderate expenditure of time. All require some 

 apparatus, which may tax a private purse too heavily, 



1. Microscopic Photography. — This is becoming abso- 

 lutely necessary to the worker who has important 

 results to communicate, and it would be a real service to 

 science if skilled amateurs could be found to aid the 

 investigator by reproducing his preparations. 



2. Preparations of Rock Sections for the Microscope. — 

 This is more difficult, and means plenty of hard work. 

 Very important results might be expected if practised 

 operators could be persuaded to work for competent 

 petrologists. 



3. Microscopic Section-cutting (with the freezing or 

 the rocking Microtome). — A short course of instruction 

 by a good hand is simply indispensable, but once learht 

 the art may be practised with great ease. 



4. Experiments on the Physiology of Plants. — Our 

 recent article on Nutrition of Plants (Scientific News, 

 page 421) gives some idea of what can be done in this 

 department. 



It would be easy to add other subjects which a work- 

 ing club could prosecute with success. But none of these 

 plans will prosper unless there are experienced persons 

 to guide the early steps, and some elementary training 

 should be insisted upon, if only as a means of finding out 

 who are really fit for special work. Our proposals will 

 not interest those who merely seek a place in the country 

 newspaper, but they may be of use to such as are trying 

 to help one another along the difficult path of practical 

 work in Natural History. 



We propose to point out from time to time under the 

 heading of " Work for Naturalists' Clubs," objects which 

 may profitably engage an actual working club. No 

 highly specialised knowledge will be required, but the 

 methods practised in an elementary course will be taken 

 for granted. 



— ■•~^>t^'^*>£-* — 



Researches on Diamagnctisni and Masine-crystallic Action. 

 By John Tyndall, D.C.L., LL.D., F.R.S. New 

 Edition. London : Longmans, Green and Co. 1888. 

 These researches, the first series of which were pub- 

 lished in 1850, are the result of the earliest experimental 

 work of one who will always be known as one of the ablest 

 experimenters of his day. As continuations of the work 

 of Faraday, they are of considerable interest to the 

 student who can afford to give time to one of the 

 branches of science which has up to the present given 



but little signs of bearing any fruit that will be of practi- 

 cal service. Faraday was one of those true men of 

 science who worked for the sake of working, and laboured 

 for the mere love of knowledge. That there are not 

 more such men is due largely to the fact that many of 

 those who have the requisite genius are compelled by 

 their circumstances to use their talents in searching for 

 new discoveries of a useful character, which will be 

 commercially valuable. Such a man, for example, was 

 James Watts. Had Faraday been compelled to devote 

 his attention to work which appeared likely to lead 

 to useful results, we certainly should not have had 

 his researches on diamagnetism, and in all probability 

 the invention of the dynamo, and scores of other 

 applications of the highest value, would have been 

 delayed. 



Diamagnetism may be briefly described thus. Bodies 

 may be divided into two classes, which are called 

 paramagnetic, or simply magnetic, and diamagnetic. 

 Iron is the most paramagnetic substance, and nickel 

 and cobalt and certain other metals are so to a 

 less degree. These substances are attracted by an 

 ordinary magnet, and a little bar will place itself 

 between the two poles of a magnet lengthwise, thus 

 allowing the magnetism to " flow through " it, to use 

 a convenient but probably inaccurate expression. Bis- 

 muth, on the other hand, is the most diamagnetic sub- 

 stance, and is feebly repelled by a magnet; a bar of 

 bismuth will place itself across the poles of a magnet. 

 Other substances, including liquids and gases, may be 

 arranged in a list, or a double list, iron heading the 

 one, and bismuth the other. Such lists may be found 

 in any text-book of electricity, but that is not a sufficient 

 reason to account for the absence of such a list from 

 the book before us, which merely refers to the lists 

 arranged by Faraday and others. The explanation 

 which Faraday gave was that a magnetic medium 

 probably exists in space, and that magnetic (or para- 

 magnetic) substances are more magnetic than this 

 medium, while bismuth and the diamagnetic substances 

 are less magnetic, and therefore exhibit opposite proper- 

 ties. A precisely analogous case is familiar to us in 

 the floating or the sinking of bodies in water. A body 

 heavier than water will sink, being attracted to the earth 

 with greater force than the surrounding water; but a 

 cork will rise, not because it is repelled by the earth, 

 but because the water is attracted more strongly. The 

 author combats this theory, and gives his reasons in a 

 letter addressed to Faraday, who was not convinced; and, 

 without disrespect to Professor Tyndall, we venture to 

 say that Faraday's explanation is still generally accepted. 

 " Magne-crystallic force," to use Faraday's own words, 

 " appears to be very clearly distinguished from either 

 the magnetic or diamagnetic forces, in that it causes 

 neither approach nor recession, consisting not in attrac- 

 tion or repulsion, but in its giving a certain determinate 

 position to the mass under its influence, so that a given 

 line in relation to the mass is brought by it into a given 

 relation with the direction of the external magnetic 

 power." It is found that most crystals, whether gener- 

 ally magnetic such as carbonate of iron, or diamagnetic 

 as saltpetre, the optic axis will set itself in a definite 

 position with respect to the poles of a magnet. In 

 carrying out experiments, the crystals are generally 

 rubbed down to the form of a ball, that any general 

 attraction of their mass may not interfere with the re- 

 sult. Numerous precautions have to be taken to ensure 



