THE SMALLEST WORKERS 



extremely difficult to get away, even in theory, from tan- 

 gible realities. When the rubbed amber acquires the 

 property of drawing the pith ball to it, we naturally 

 assume that some change has taken place in the con- 

 dition of the amber; and since the visible particles of 

 amber appear to be unchanged since its color, weight, 

 and friability are unmodified it seems as if some im- 

 material quality must have been added to, or taken from 

 it. And it was natural for the eighteenth-century 

 physicist to think of this immaterial something as a 

 fluid, because he was accustomed to think of light, heat, 

 and magnetism as being also immaterial fluids. He did 

 not know, as we now do, that what we call heat is 

 merely the manifestation of varying " modes" of motion 

 among the particles of matter, and that what we call 

 light is not a thing sui generis, but is merely our recog- 

 nition of waves of certain length in the all-pervading 

 ether. The wave theory of light had, indeed, been pro- 

 pounded here and there by a philosopher, but the theory 

 which regarded light as a corpuscular emanation had the 

 support of no less an authority than Sir Isaac Newton, 

 and he was a bold theorist that dared challenge it. 

 When Franklin propounded his theory of electricity, 

 therefore, his assumption of the immaterial fluid was 

 thoroughly in accord with the physical doctrines of the 

 time. 



MODERN VIEWS 



But about the beginning of the nineteenth century 

 the doctrine of imponderable fluids as applied to light 

 and heat was actively challenged by Young and Fresnel 



[153] 



