Magnefism, Cohesion, Adhesion and Viscosity, -t^l 



by some substances, but also for the loss of this quality, that 

 in many cases where the needle was with difficulty made to 

 attach itself, on contact being broken a fresh connexion was 

 produced with the greatest ease, if only a short delay had 

 taken place, and no change whatever had been made in the 

 position of the object. Time, too, was still more necessary to 

 the success of the experiments when the angles were to be 

 measured ; for it not unfrequently happened that, when I had 

 supposed the greatest amplitude had been attained, on main- 

 taining the forced connexion for a i&w minutes with an enlarged 

 angle, a firm connexion took place; thus throwing a doubt 

 upon the measures I had previously noted as the highest at- 

 tainable in other substances. Such measures, therefore, even 

 in any case, can only be considered as approximative, as 

 greater deviations may possibly be yet obtained by myself or 

 other observers. Thus in the case of a spark of diamond, I 

 was able to increase the angular distance from 42° to 65°. So 

 likewise in the case of quartz, it rose gradually from 45° to 

 90°. Now when it is considered that the attachment of the 

 needle takes place generally with the greatest readiness when 

 the angle of deviation is small, say 10° or 15°, owing to its 

 limited deviation from the meridian in such cases, it can leave 

 but little doubt that this peculiarity arises from the slow effect 

 of magnetic propagation in the substance examined, in which, 

 to use language that has become familiar in describing mag- 

 netic phaenomena, saturation takes place but slowly. 



The foregoing facts entirely accord with the remark made 

 by Sir John Herschel and Mr. Babbage, when in 1825 they 

 varied the experiments of M. Arago. A remarkable confir- 

 mation of this peculiarity was exhibited by chromium, which 

 was entered, after many trials with a magnetic needle, as a 

 failure, yet on repeating the experiment, and allowing a long 

 time for the needle to operate, the measurements rose succes- 

 sively, according to the notes taken on the occasion, as follows : 

 20°, 25°, 30°, 40^, 45°, 49°, 55°, 70°, 90°. The notes on the 

 subject having been overlooked, a fresh attempt at measure- 

 ment, after the lapse of more than a year, was recently made, 

 when it was again entered as a failure, as there was not the 

 least tendency to a connexion between the needle and the 

 chromium after the ordinary time given to other difficult sub- 

 stances. 



On reflecting on the necessity of time being given to deve- 

 lope the magnetism of some substances, and remembering that 

 the needle, which was the active agent in its production, did 

 not weigh four grains, and that the parts of the needle and the 

 substance in cojitact were merely two minute points, it struck 



