1870.] Notices of Scientific Works. 503 



Professor Tyndall concludes that a body of evidence has accumulated 

 in favour of diamagnetic polarity, which places it among the most 

 firmly-established truths of science. 



This being the case, it would be of interest to ascertain on 

 which side exists the fallacy of reasoning by which Professor 

 Thompson has reduced the existence of diamagnetic polarity to an 

 apparent absurdity ; the paradox is well stated in the following quo- 

 tation from a paper by Faraday, " On some Points of Magnetic 

 Philosophy," published in the ' Philosophical Magazine ' for Feb- 

 ruary, 1855 : — " If a globe of bismuth be placed without friction in 

 the middle of the magnetic field, it will not point or move because 

 of its shape ; but if it have reverse polarity, it will be in a state of 

 unstable equilibrium ; and if time be an element, then the ball, 

 being once moved on its axis ever so little, would then have its 

 polarity inclined to the magnetic axis, and would go on revolving 

 for ever, producing a perpetual motion. I do not see how this con- 

 sequence can be avoided, and therefore cannot admit the principles 

 on which it rests. The idea of a perpetual motion produced by 

 static forces is philosophically illogical and impossible, and so I 

 think is the polarity opposed or adverse static condition to which 

 I have already referred." 



Of course if time does not enter as an element in diamagnetic 

 induction the above argument falls to the ground ; but it appears 

 to be so firmly established a fact that an exertion of physical force 

 occupies time, that it can scarcely be doubted that it is concerned 

 here also ; that was Faraday's opinion, although he admitted that 

 it seemed to be so brief in period as to be inappreciable by the 

 means he had employed. 



We should have liked to give an extended notice of the second 

 subject included in the title of this work, namely, Magne-crystallic 

 action, — the phenomena of which were at first so paradoxical as to 

 baflle the ingenuity of the most acute experimentalists, but, thanks 

 to the labour of Professor Tyndall and other physicists, now dedu- 

 cible with as much care and certainty from the action of polar forces 

 as the precession of the equinoxes is from the force of gravitation. 

 In the author's language, " The whole domain of magne-crystallic 

 action is thus transferred from a region of mechanical enigmas to 

 one in which our knowledge is as clear and sure as it is regarding 

 the most elementary phenomena of magnetic action." 



The magne-crystallic force is one by which certain crystals are 

 caused to set themselves with certain of their axes parallel or trans- 

 verse to the lines of magnetic force acting on them. This force acts 

 at a distance, and is by no means so weak as might be at first sup- 

 posed, for just as a crystal is moved by the magnet at a distance, so 

 can the crystal also move the magnet at a distance. Faraday ob- 

 tained the latter result by converting a steel bodkin into a magnet 



