Transparent Bodies by the Action of Magnetism. 419 



produce a more feeble rotation than that of water, while the 

 third exerts a more powerful one *. Nevei'theless the extreme 

 weakness of the magneto-rotary power of concentrated solutions 

 of protochloride of iron, taken in conjunction with the obsei'- ' 

 vations of M. Bertin on the protosulphate of iron, appeared to 

 me to indicate the existence of a particular mode of action, at 

 least with compounds of iron, if not with all magnetic com- 

 pounds, worthy of more minute examination. 



Such examination has formed the principal object of my in- 

 vestigation, the results being those which I have obtained on 

 examining in succession the compounds of the different mag- 

 netic metals. 



Iron. — When a salt of the protoxide of iron is dissolved in 

 water, the solution has always a magneto-rotary power weaker 

 than that of water, and less according as the solution is stronger. 

 But more than this : if, by taking account of the density and 

 composition of the liquid, the rotation be calculated which would 

 be produced by the water alone contained in a given volume of 

 the solution, a number is found always greater than the ob- 

 served one. It would therefore appear as if the salt of iron 

 dissolved, exercised on the polarized light an action opposite to 

 that of water. Thus a solution of protochloride of iron of den- 

 sity 1'2922, and containing 72"3 per cent, by weight of water 

 and 28*3 of protochloride, effects under the influence of mag- 

 netism a rotation of 0'581, if the rotation produced by water be 

 taken as unity. It follows from the above numbers, that one 

 unit volume of the solution contains a proportion of water equal 

 to 0-9265 of the weight unit, and that consequently if the salt 

 dissolved were completely passive, the rotation should have been 

 precisely 0'9265. Inasmuch as it is considerably less, it is 

 natm-al to suppose that the salt dissolved effects a rotation 

 opposite to that of water, and equal in absolute value to the 

 difference between 0-926 and 0-58], that is, equal to 0-345. 



All the salts of protoxide of iron which I have examined have 

 furnished me with similar results. Taken in conjunction with 

 the law established above concerning non-magnetic solutions, it 

 is clear that salts of protoxide of iron, when subjected to mag- 

 netic influence, exert an action on polarized light contrary to 

 that exercised by water, sulphide of carbon, glass and most trans- 



* M. Edmond IJecquercl does not appear to have considered the in- 

 fluence which the coloration of Uquids exerts upon the determination of tlic 

 passage-tint. This influence, which is almost nothing in the case of salts 

 of protoxide of iron, is very great with salts of nickel,— a fact which made 

 me at first imagine that, by allowing for it, the salts of nickel would be 

 brought under the general law enunciated by M. Becquerel. This is by 

 no means the case ; we shall suhsecjuently see that solutions of nickel salts 

 have really a rotary magnetic power greater than that of water. 



