428 ACTUAL WORKING IN ORIGINAL RESEARCH. 



from one rail through the ball to the other, the ball 

 rotated, and ran round-the circular railway as long as the 

 current continued. In this case the electric current was 

 the cause of the motion, but only indirectly, the direct cause 

 being expansion produced by the heat of electric conduc- 

 tion-resistance. The current produced heat at the points 

 of contact of the ball and rails, and the heat produced 

 expansion of those parts into small prominences. During 

 the fraction of time required to produce these effects, the 

 started ball, by its momentum, moved forward a minute 

 distance, and thus the prominences were always a little 

 behind the centre of gravity of the ball, and pushed the 

 ball forwards. 3. The phenomenon B may be the only 

 direct result of A ; this is a rare case : or 4. It may be, 

 as commonly occurs, only one of the direct results, others 

 being coincident with it ; thus an electric current passed 

 through an iron wire not only produces a molecular change, 

 sound, and heat in the wire, but also magnetic influence 

 around it. 5. B may be composed of two portions, one of 

 which is directly due to A, and the other to a second and 

 coincident cause A' ; thus the heat produced by burning a 

 jet of hydrogen is partly due to the chemical union of the 

 two substances, and partly to the latent heat set free by 

 the condensation of the product of combustion. 6. Or B 

 may be an effect (usually one of several) of the combined 

 action of two conditions, neither of which alone is capable 

 of producing it in the slightest degree ; thus a flame 

 applied to hydrogen alone or to oxygen alone produces no 

 explosion, but if applied to a mixture of the two, produces 

 it. 7. B may also be a result of many concurring con- 

 ditions, and does not take place in the least degree unless 

 they are all present. Instances of this kind occur more 

 frequently in the concrete sciences of animal life than in 

 the simpler ones relating to inorganic matter ; for in- 



