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 
hematite! 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,S ), with 
rather less than 4o per cent. of sulphur, was highly magnetic, 
while ordinary pyrite (FeS,), 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 magnétique, cepen- 
dant les micas noirs-brunatres le sont assez pour adhérer 
facilement a l’électro-aimant. Je me suis méme servi de cette 
propriété pour déterminer la proportion de mica qui se trouve 
dans quelques granites de la Normandie et de la Bretagne qui 
sont employés au pavé de Paris.” ”) 
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 Fouqué published in the Comptes rendus (tome 
Ixxv.) an abstract of a memoir entitled “ Nouveaux procédés 
d’analyse médiate des roches et leur application aux laves de 
la derniére eruption de Santorin.” * 
1 Excluding those highly magnetic crystals which contain ferrous oxide, 
probably due to intergrowths of magnetite. 
2 Annales des Mines, 1848, tome xiv. p. 479. 
3 For the full memoir see Mém. prés. par divers savants a4 PAcadémie des 
Sciences, tome xxii. No. 11, 1876. 
