463 THE PEINCIPLES OF SCIENCE. [chap. 



equivalent of heat, disregarded the heat absorbed by the 

 apparatus, otherwise he would, in Dr. Joule's opinion, have 

 come nearly to the correct result. 



It is surprising to learn the number of causes of error 

 which enter into the simplest experiment, when we strive 

 to attain rigid accuracy. We cannot accuiately perform 

 the simple experiment of compressing gas in a bent tube 

 by a column of mercury, in order to test the truth of 

 Boyle's Law, without paying regard to — (i) the variations 

 of atmospheric pressure, which are communicated to tlie 

 gas through the mercury ; (2) the compressibility of 

 mercury, which causes the column of mercury to vary 

 in density ; (3) the temperature of the mercury through- 

 out the column; (4) the temperature of the gas, which is 

 with difficulty maintained invariable ; ($) the expansion 

 of the tilass tube containing the sjas. Althouc^h Eesnault 

 took all these circumstances into account in his examina- 

 tion of the law,^ there is no reason to suppose that he 

 exhausted the sources of inaccuracy. 



The early investigations concerning the nature of waves 

 in elastic media proceeded upon the assumption that 

 waves of different lengths would travel with equal speed. 

 I^ewton's theory of sound led him to this conclusion, and 

 observation (p. 295) had verified the inference. When 

 the undulatory theory came to be applied at the com- 

 mencement of this century to explain the phenomena of 

 light, a great difficulty was encountered. The angle at 

 which a ray of light is refracted in entering a denser 

 medium depends, according to that theory, ou the velo- 

 city with which the wave travels, so that if all waves 

 of light were to travel with equal velocity in the same 

 medium, the dispersion of mixed light by the prism and 

 the production of the spectrum could not take place. 

 Some most striking phenomena were thus in direct con- 

 flict with the theory. Cauchy first pointed out the ex- 

 planation, namely, that all previous investigators had made 

 an arbitrary assumption for the sake of simplifying the 

 calculations. They had assumed that the particles of the 

 vibrating medium are so close together that the intervals 

 are inconsiderable compared with the length of the wave. 



1 Jamiu, Cours de Physique, vl. i. [»p. 282, 283. 



