254 Magnetic Influence of Solar Light, 



in imparting magnetism under the circumstances described. 

 That the differences observable in the periods of Oscillation 

 before and after exposure are due, at least in part, to 

 minute and unavoidable differences in the conditions of 

 experiment for each needle, I have but little hesitation in 

 affirming, but their extent and uniformity of direction for- 

 bid their being attributed wholly to such a cause, and warrant 

 a remark, which I am not aware of having been previously 

 made, namely, that brightly polished soft steel needles, after 

 being exposed for a certain time to the direct light of the 

 sun, and subsequently withdrawn to the shade, will vibrate in 

 sensibly longer periods than before exposure. That this 

 result is not magnetic in its origin, is, I think, very clearly 

 proved by the following experiments made with the express 

 view of testing this point. An exceedingly slender sewing 

 needle, about one inch in length, and one hundredth of an 

 inch in diameter, entirely devoid of all magnetism, was sus- 

 pended within a glass shade by means of a single filament 

 of silk, about six or seven inches in length, forming a very 

 delicate and sensitive testing apparatus for magnetic polari- 

 ties. To this testing needle, each of the cylinders A B and 

 C was in turn approximated, but not the slightest indica- 

 tion of polar action of any kind could be obtained, the 

 needle being utterly indifferent to the presence of the 

 cylinders. This result, both serves to verify that given by 

 the different method of oscillations, and to shew that to 

 whatever cause the longer periods of oscillation after expo- 

 sure are to be attributed, there is no reason to believe that 

 magnetism is concerned in producing them. That the 

 action of the sun is capable of producing remarkable 

 changes in the internal constitution of bodies, while their 

 external appearance continues unaffected, has been very 

 strikingly established by the experiments of M. Mitscherlich 

 of Berlin, who has found, among other instances, that pris- 

 matic crystals of the metal zinc are completely changed 



