286 Dr. Tyndall on the Laws of Magnetism. 



experiments by placing one leaf between the poles, and deter- 

 mining tlie weight necessary to separate the magnets. A second 

 leaf is then introduced, and the separating weight again detei'- 

 mined. The experiments are continued until a distance of 46 

 leaves is attained. The author then subtracts each weight from 

 that next preceding, and in the column of numbers thus obtained 

 he observes the irregularity complained of. M. Cramer's expe- 

 riments, however, seem to prove decisively, that the law above 

 developed for a sphere of soft iron and a straight core, magnet- 

 ized by induction, is also true for two steel magnets of the horse- 

 shoe form. Commencing at the 5th leaf in Table II., and mul- 

 tiplying the distances by the weights corresponding to them, we 

 obtain the following series of numbers : — 



The products here may be regarded as constant, as the slight 

 and irregular differences which they exhibit are evidently due to 

 the mode of experiment. ' " ' ' 



§7. 



27. Bodies capable of magnetization are divided into two 

 classes, in one of which the magnetic force is readily aroused, but 

 as quickly lost again when the exciting cause is removed ; the 

 other class, on the contrary, accepts the magnetic state with dif- 

 ficulty, bvit retains it when once excited. Soft iron is an example 

 of the one class, steel is an example of the other. When a piece 

 of soft iron is acted upon by a magnet, it is said to be magnet- 

 ized by influence. The magnet in this case is, so to speak, the 

 creator of the force which responds to its attraction. At the 

 commencement of this inquiry we arrived at the notion of the 

 strength of a magnet from its action upon a freely suspended 

 magnetic needle. The motion of the needle is the result of 

 reciprocal action ; but the force with which the needle reacts 

 upon the magnet is not the gift of the latter, as in the case of 

 soft iron. Adhering to the common notion of a magnetic fluid, 

 in the case of soft iron the magnet decomposes and disposes of 

 this fluid so as to cause mutual attraction. In the needle, the 

 fluid is, so to speak, fitted by previous treatment for the action 

 of the magnet. The steel, in the case before us, resists mag- 

 netization by influence; and the magnet acts upon an indepen- 

 dent fluid, without either increasing or diminishing the quantity 

 thereof. Were this otherwise, tan « (7.) would not be a proper 

 measure of magnetic power. Let us suppose the case of a little 



