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IX. Experimental Researches in Electricity. — Thirtieth Series. By Michael 
Faraday, Esq., D.C.L., F.R.S., Fullerian Prof. Chem. Royal Institution, Foreign 
.Associate of the Acad. Sciences, Paris, Ord. Boruss. Pour le Mirite, Eq., Memb. 
Royal and Imp. Acadd. of Sciences, Petershurgh, Florence, Copenhagen, Berlin, 
Gottingen, Modena, Stockholm, Munich, Bruxelles, Vienna, Bologna, Commander 
of the Legion of Honour, Sfc. Sfc. 
Received October 24, — Read November 15 and 22, 1855. 
§ 38. Constancy of differential magnecrystaUic force in dfferent media. 
§ 39. Action of heat on magnecrystals. 
§ 40. Effect of heat upon the absolute magnetic force of bodies. 
3363. Whilst using- lines of force as a true, searching, and as yet, never-failing 
representative of the one form of power possessed by paramagnets, diamagnets, and 
electric currents, — and whilst endeavouring simultaneously to make the principle of 
representation a key to new phenomena, and subjecting the principle itself to rigid 
cross examination, — I have had occasion to examine the action of certain magnetic 
bodies in different media and at different temperatures ; and as the results are true, 
and must, therefore, be valuable in any view of the cause of magnetic action, I 
have thought them worthy of presentation to the Royal Society. 
3364. When an unmagnetized but magnetic body, placed in a magnetic field, is 
affected by the forces thrown upon it, and under their action sets, or takes up a 
definite position, the effect may depend upon its peculiar molecular condition, or 
upon its relation to the surrounding medium, or upon both conjointly, or upon one 
or both combined with temperature. Some of each of these conditions have been 
the objects of my investigations. 
§ 38. Constancy of differential magnecrystaUic force in dfferent media. 
3365. When a sphere or cylinder formed from a crystal of calcareous spar or of 
bismuth is suspended in a particular direction in the magnetic field, it points with 
considerable power, whereas if formed out of amorphous or granular carbonate of 
lime or bismuth, it has no such tendency. In the latter case, it (by reason of its 
relation to the surrounding medium) is urged to move from stronger to weaker places 
of force ; in the former case, also, it has the same tendency, but the power of point- 
ing, which it possesses in addition, has no relation to the medium about it, but only 
to the difference in strength of the magnetic force, as developed in different directions 
within the sphere itself. Such effects constitute the branch of science known under 
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