38 SCIENCE PROGRESS 



the low magnetic power of the chlorites to their state of 

 hydration, containing as they do about 12 per cent, of water. 

 In this connection we may note that the contrast between 

 haematite * and limonite is a good example of the effect of 

 hydration in diminishing the magnetic power of a mineral. 

 Another example of this is copper sulphate, which is distinctly 

 more magnetic in the anhydrous than it is in the hydrated 

 condition. 



Delesse attributed the low magnetic power of black tourma- 

 line to the diamagnetic effect of boron. He observed that the 

 magnetic power of sulphides diminished very rapidly as the 

 percentage of sulphur increased : thus, pyrrhotite (Fe n S ), with 

 rather less than 40 per cent, of sulphur, was highly magnetic, 

 while ordinary pyrite (FeS 2 ), with about 54 per cent, of sulphur, 

 was practically non-magnetic. But here, again, we are tempted 

 to remember that sulphur is a diamagnetic element. 



Before quitting this consideration of Delesse's work, it is 

 important to remark that he crushed certain French biotite- 

 granites, and, by the electromagnetic method, not only isolated 

 the biotite, but estimated quantitatively its proportion to the 

 bulk of the rock. (" Le mica est faiblement magnetique, cepen- 

 dant les micas noirs-brunatres le sont assez pour adherer 

 facilement a l'electro-aimant. Je me suis meme servi de cette 

 propriete pour determiner la proportion de mica qui se trouve 

 dans quelques granites de la Normandie et de la Bretagne qui 

 sont employes au pave de Paris." 2 ) 



This appears to be the first record of the use of the electro- 

 magnet in the isolation of feebly magnetic minerals from rocks, 

 and the quantitative estimation of the same. But the method 

 is evidently only a logical extension of that adopted by the 

 earlier workers, Bellevue and Cordier, for the separation of 

 magnetite. 



In 1872 Fouque published in the Comptes rendus (tome 

 lxxv.) an abstract of a memoir entitled " Nouveaux procedes 

 d'analyse mediate des roches et leur application aux laves de 

 la derniere eruption de Santorin." 3 



1 Excluding those highly magnetic crystals which contain ferrous oxide, 

 probably due to intergrowths of magnetite. 



2 Annales des Mines, 1 848, tome xiv. p. 479. 



3 For the full memoir see Mem. prcs. par divers savants a PAcade'rnie des 

 Sciences, tome xxii. No. 11, 1876. 



