112 Trans. Acad. Sci. of St. Louis. 



spaces, having perhaps an increasingly higher order of magni- 

 tude? 



The determination of the amount of heat radiated from the 

 sun, per second, per unit of area, was attempted by Pouillet, 

 and has been more accurately determined by Langley. This 

 result has served as a basis for the determination of the dura- 

 tion of the life of our universe. There is some ground for 

 thinking that this time interval may be determined with 

 reasonable precision, but it is doubtful if such estimates can 

 as yet receive much weight. 



But a study of the flow of heat from the interior of the 

 earth has enabled Sir William Thomson to determine between 

 reasonable limits the interval since the earth began to solidify. 

 The information needed for this is, the rate of increase in 

 temperature with depth, and the conducting power of the 

 material forming the outer shell of the earth. This result 

 has attracted keen attention from geologists, for the interval 

 found is much shorter than that formerly thought necessary 

 to accomplish the work of geological time. 



Another great step has been the increase in our knowledge 

 of the ether. When the existence of an ether which filled all 

 space was suggested, it was a conjecture based on possibility. 

 The logical situation involved a choice between two theories 

 of light. Newton had suggested that light might be a dis- 

 charge of particles which shoot off from all luminous bodies, 

 and which must travel with enormous speed. This condition 

 has been strongly simulated in an artificial way in the interior 

 of the Crookes tube. The cathode discharge falling upon the 

 walls of the tube arouses the X-ray into activity, just as 

 Newton thought the luminous particles might bombard the 

 retina and arouse the sensation of light. 



Many of Newton's followers were dogmatic in their adher- 

 ence to his ideas. He was not. 



Newton's ideas were held by a majority of the great men at 

 the beginning of the century. But in the merciless examina- 

 tion which was given it the emission theory was found inade- 

 quate. At the beginning of the century Brewster was one of 

 the foremost exponents of optics in England, and he strongly 

 condemned the wave theory of light. He has placed upon 



