446 THE PRINCIPLES OF SCIENCE. [chap. 



placing bright reflective buttons on the tops of wires of 

 various forms. The motions are performed so quickly that 

 the eye receives the impression of the path as a complete 

 whole, just as a burning stick whirled round produces a 

 continuous circle. The laws of electric induction are 

 beautifully shown when iron filings are brought under the 

 influence of a magnet, and fall into curves corresponding 

 to what Faraday called the Lines of Magnetic Force. 

 AVhen Faraday tried to define what he meant by his lines 

 of force, he was obliged to refer to the filings. " By mag- 

 netic curves," he says,^ " I mean lines of magnetic forces 

 which would be depicted by iron filings." Eobison had 

 previously produced similar curves by the action of fric- 

 tional electricity, and from a mathematical investigation of 

 the forms of such curves we may infer that magnetic and 

 electric attractions obey the general law of emanation, 

 that of the inverse square of the distance. In the electric 

 brush we have a similar exhibition of the laws of electric 

 attraction. 



There are several branches of science in which collective 

 experiments have been used with great advantage. Lich- 

 tenberg's electric figures, produced by scattering electrified 

 powder on an electrified resin cake, so as to show the con- 

 dition of the latter, suggested to Chladui the notion of 

 discovering the state of vibration of plates by strewing 

 sand upon them. The sand collects at the points where the 

 motion is least, and we gain at a glance a comprehension 

 of the undulations of the plate. To this method of experi- 

 ment we owe the beautiful observations of Savart. The 

 exquisite coloured figures exhibited by plates of crystal, 

 when examined by polarised light, afford a more compli- 

 cated example of the same kind of investigation. They 

 led Brewster and Fresnel to an explanation of the properties 

 of the optic axes of crystals. The unequal conduction of 

 heat in crystalline substances has also been shown in a 

 similar manner, by spreading a thin layer of wax over the 

 plate of crystal, and applying heat to a single point. The 

 wax then melts in a circular or elliptic area according as 

 the rate of conduction is uniform or not. Nor should we 

 forget that Newton's rings were an early and most impor- 



' Faraday's Life, by Bence Jones, vol. ii. p. 5. 



