1835.] BecquereVs Traite Experimental, Sfc. 151 



by suspending them (after being cylindrically formed) by means of 

 a thread, applying a needle, and judging of the magnetic force by 

 the number and rapidity of the oscillations, and he extended this 

 method to detecting small quantities of iron disseminated through 

 greater masses of other metals. Biot employed it likewise to detect 

 iron in minerals, as for instance, in two different species of mica, one 

 from Siberia and the other from Zinwald, in Bohemia. The former 

 executed 7 oscillations in 55 seconds, the latter 12 in the same time. 

 The magnetic forces of each were as the squares of these numbers, or 

 49 to 144 ; considering these forces as proportional to the quantity 

 of oxide of iron., the Zinwald mica should contain 20 per cent, of the 

 oxide, and the other 6*8, which corresponds with the result of 

 analysis. Haiiy endeavoured to detect still smaller quantities of 

 iron by a modification of this plan. By these means it has been 

 ascertained that all bodies placed near very powerful magnets mani- 

 fest feeble magnetic properties attributable to small quantities of iron 

 which have not been detected by art. Arago has proved further that 

 all bodies in the neighbourhood of a needle which oscillates, produce 

 in it an action, the effect of which is to diminish the extent of the 

 oscillations without diminishing their number. He found also that 

 by causing a rotatory motion in a plate of copper, placed under a 

 magnetic needle, that the needle was driven from the magnetic 

 meridian at the commencement of the rotation, increasing in force 

 proportionally to the rapidity. Prevost and Colladon deduced from 

 their experiments that the angles of deviation, and not their signs, 

 increase in proportion to the rapidity that the signs of the angles of 

 deviation increase inversely with the power 2j of the distance. 



Babbage and Herschel have announced that the law is not con- 

 stant, and that it varies between the square aud the cube of the 

 distance. 



Barlow observed, that whatever be the direction of the axis of 

 rotation, if the movement of the rotating body is directed towards 

 the needle, the north pole of the latter is attracted ; if the contrary, 

 then the extremity is repulsed. If the needle be carried round the 

 rotating body parallel to the axis, it has a tendency to arrange itself 

 at right angles with it. 



1 1. Electricity and magnetism although they agree in most respects 

 differ apparently in this, that electricity penetrates into all substances, 

 while magnetism only enters three bodies in a state of rest, viz. iron, 

 cobalt and nickel. Poisson has endeavoured to reconcile this and the 

 other facts with which we are acquainted in reference to this prin- 

 ciple, by his theory. He conceives that we may represent a mag- 

 netic body, as a collection of magnetic parcels separated by spaces 

 inaccessible to magnetism. The relation of the sum of all these par- 

 cels to the entire volume of the body which may be considered its 

 density, in relation to magnetism, will be a fraction which will ap- 

 proach unity more or less in bodies of a different nature, and which 

 ought to be given for each body in particular. He attributes all the 

 magnetic phenomena to two imponderable fluids, influenced by 

 general laws of equilibrium and motion, and which may exercise upon 

 bodies, in consequence of the reciprocal action of their particles, pres- 



