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for the Advancement of Science. 267 



theoretical calculations of Cauchy, founded on a molecular hypothesis 

 applied to the undulatory theory. And (astly, some curious experi- 

 ments by Mason, Jamin, Prevostaye, and Desains, appear to show more 

 fully, what had partially been shown by Prof. Forbes, that radiant heat 

 admits of polarization in all respects similar to that of light. * 



* « 



2. 



id Magne'Cryslallic Action; by Dn 

 J. Tyndall- 



One of the most important inquiries which at the present day occupy 

 the attention o( the student of physical science, is the relation which 

 subsists between magnetism and diamagnetism. Are the laws which 

 govern both forces identical ? Will ihe mathematical expression of the 

 attraction in the one case be converted into that of the repulsion in the 

 other case, by a change of sign from positive to negative? To this 

 question Plucker replies '^ No." His experiments have led him to the 

 conclusion, that when the power of a magnet which operates upon a 

 body composed of magnetic and diamagnetic constituents is increased, 

 the diamagnetism of the compound mass increases in a much quicker 

 ratio than the magnetism ; that in consequence of this an indifferent 

 body is a physical impossibility ; for a body in which the respective for- 

 ces might be exactly equal and apposite when excited by a magnet of 

 a certain strength would, upon lowering the power of the magnet be- 

 low this standard, be attracted, — and by increasing the power of the 

 magnet beyond this standard, be repelled. 



During a previous investigation, the author of the present memoir 

 had repeated opportunities of observing phenomena exactly similar to 

 some of those which form the premises of Plijcker's conclusion ; and a 

 close study of the subject convinced him that to account for these phe- 

 nomena the hypothesis of two conflicting forces in the same compound 

 mass, the one or the other of which predominates according as the 

 power of the magnet is increased or diminished, was by no means ne- 

 cessary. To fit himself for the investigation of this question, he com- 

 menced an inquiry last November into electro-magnetic attractions; 

 one of ihe results of this inquiry was, that a sphere of soft iron sepa- 

 < rated from the end of a straight electro-magnet by a small fixed dis- 



tance, was attracted by the latter with a force exactly proportional to 

 the square of the exciting current. Now, this attraction is in each case 

 the product of two factors, one of which expresses the magnetism oi 

 the magnet, and the other the magnetism of the ball ; and it is easy to 

 see, that while the attraction increases as the square of the current, the 

 magnetism of the ball increases in the simple ratio of the current it- 

 felf. Our way to a comparison of magnetic attraction and diamagnet- 

 ic repulsion is now clear. We know the law according to which the 

 magnetism of the iron ball increases, and we have only to mquire 

 ^hether the diamagnetism of the bismuth ball follows the same lavv. 

 The apparatus used in the former case proved, however, to be totally 

 "nfit for the measurement of diamagnetic force,— the feebleness of the 

 latter rendered a much more delicate mode of measurement necessary. 

 The torsion balance was the instrument finally resorted to by the au- 

 thor. A loop of paper was attached to one end of a fine silver wire, 

 and in the loop rested a little beam of light wood. At the ends of the 



