6 THOUGHTS ON OUR CONCEPTIONS OF PHYSICAL LAW, 



Prof. Jevons appears to think tl^at our difficulty in such cases, is due to an 

 imperfect idea of infinite space.* 



In the study of Physics, our most certain experimental results force us to 

 ideas equally beyond our power of realization. It is shown beyond question, 

 that light moves over a distance of about seven times the circumference of our 

 earth in a single second. We must look for something marvelous in any theory \ 

 which can account for so marvelous a fact. According to Newton's theory, we 

 should have particles of light, shooting off from a distant luminous body with j 

 this immense velocity, and, falling upon a mirror, their motion would not merely 

 be checked, but the elasticity of these light particles must be assumed to be so 

 perfect, that they rebound with an equal velocity. 



According to the undulatory theory, the light consists of vibrations of a 

 medium which fills all space. Since the velocity of transmission of these vibra- 

 tions is so great, it follows that the elasticity of this medium must be 10,000,000,000 

 times as great as that of the hardest steel. Space is not now regarded as a void, 

 but as filled with a medium which, as Thomas Young remarked, *' is not only highly 

 elastic, but absolutely solid." And yet, as we walk through space, the solid 

 atoms which compose our bodies, experience not the slightest resistance. Such 

 ideas, although they can be .conceived, cannot be realized. We have had 

 no previous experience with materials possessing such properties, and such ideas 

 must necessarily appear strange to us ; but they are no more strange than the 

 phenomena of light which we directly observe, and which force us to this, or to 

 some other theory, equally marvelous. Only those who have carefully examined 

 the subject, can realize how weighty is the evidence in favor of the undulatory 

 theory of light ; but where such stupenduous conceptions are involved, a slavish 

 acceptance of any theory, even by them, would be in the highest degree objec- 

 tionable. We are not the friends of theories, but of truth. 



So in all departments of thought, we come sooner or later to depths which 

 the human sounding line cannot pierce ; we reach ideas, about which it becomes 

 hazardous to talk, unless one courts the position of a babbler of nonsense; we 

 learn that all our " final " formulae contain unknown quantities. As we are 

 not infallible, we must therefore be cautious and modest. 



It is not surprising then, that in the progress of our sciences, many errors 

 of reasoning and in the interpretation of facts have been committed. You 

 are all familiar with the ideas of Newton, in regard to the nature of light, 

 ideas which were not in themselves absurd, which were firmly believed in 

 by this man of such transcendent power, but which were clearly negatived by 

 results of subsequent experiment. 



It was known long ago, that rain-gauges placed above the surface of the 

 ground, caught less rain than those placed at the surface, and it is still taught in 

 many of our text-books, that this is due to a condensation of moisture in the 

 lower strata of the atmosphere. This idea is not absurd, but it has been shown* 

 that this cause produces no appreciable effect, and that the observed effect is due 



* Principles of Science, p. 7C7 



