Transparent Bodies by the Action of Magnetism. 421 



cation, because they have the advantage of reminding us of the 

 direction of rotation. Water, sulphide of carbon, glass and 

 other bodies whose magneto-rotary power is positive, turn the 

 plane of polarization in the direction in which the positive cur- 

 rent of electricity traverses the conducting wire of the electro- 

 magnet : protosalts of iron and analogous bodies turn the plane 

 of polarization in the direction in which the negative current 

 passes through the same wire. It is scarcely necessary to men- 

 tion, that in the numerical Tables these two kinds of rotary 

 powers are denoted by + and — . 



However concentrated the solutions were of the protosalts of 

 iron, I only observed a greater or less diminution in the rotation 

 of the plane of polarization : a solution of the protochloride, 

 concentrated to the crystallizing point, show^ed a magneto-rotary 

 power absolutely nothing ; but I have not succeeded in effecting 

 an entire reversion of the rotation. I had hopes of succeeding 

 in this, and of thus directly demonstrating the negative rotary 

 powers of protosalts of iron, in seeking for some solid compound 

 rich in the protoxide of iron, and either uncrystallized or be- 

 longing to the cubic system of crystallization. Red garnet, 

 which may contain as much as 30 per cent, of protoxide of iron, 

 appeared suitable for the verification of this conjecture ; but 

 among numerous pieces examined, not one was found sufficiently 

 free from flaws for experiment. 



The magneto-rotary power of salts of the peroxide of iron, 

 like that of the protoxide salts, is negative; but it is much 

 greater and more easily shown than that of the protosalts. 

 Perchloride of iron especially gives very remarkable results. A 

 very dilute aqueous solution of this salt has a magneto-rotary 

 power less than that of water. According as the solution is 

 concentrated the rotary power diminishes, becomes nothing, and 

 finally changes sign : after the change of sign the power in- 

 creases up to the maximum of concentration. Thus, a solution 

 near the maximum, containing 40 per cent, of perchloride, 

 exerts an action on polarized light contrary to that of water, 

 and from six to seven times as great ; that is, nearly equal to 

 the action of pieces of Faraday's heavy glass which I have 

 examined. 



Negative rotations may be still more readily obtained by sub- 

 stituting for water other solvents which, by themselves, have 

 less action than water on polarized light, or which dissolve a 

 greater quantity of the salt. Alcohol and retlier satisfy the first 

 condition, and are consequently very well adapted for showing 

 the negative rotary power of persalts of iron. Thus, on dis- 

 solving in 80 j)art3 of a;ther 20 ])arts of crystallized chloride 

 of iron, prepared by the direct action of chlorine on iron, I ob- 



