PRIMARY CONCEPTS OF MODERN SCIENCE. 711 



sique Genkrale (ed. Moigno, Paris, 1868, p. 38), after defining atoms 

 as " material points without extension," uses this language : " Thus, 

 this property of matter which we call impenetrability is explained, 

 when we consider the atoms as material points exerting on each other 

 attractions and repulsions which vary with the distances that separate 

 them. . . . From this it follows that, if it pleased the author of Na- 

 ture simply to modify the laws according to which the atoms attract 

 or repel each other, we might instantly see the hardest bodies pene- 

 trate each other " (that we might see), " the smallest particles of matter 

 occupy immense spaces, or the largest masses reduce themselves to 

 the smallest volumes, the entire universe concentrating itself, as it 

 were, in a single point." 



2. The second fundamental proposition of the modern atomic 

 theory avouches the essential discontinuity of matter. The advocates 

 of the theory affirm that there is a series of physical phenomena 

 which are inexplicable, unless we assume that the constituent par- 

 ticles of matter are separated by void interspaces. The most notable 

 among these phenomena are the dispersion and polarization of light. 

 The grounds upon which the assumption of a discrete molecular 

 structure of matter is deemed indispensable for the explanation of 

 these phenomena may be stated in a few words. 



According to the undulatory theory, the dispersion of light, or its 

 separation into spectral colors, by means of refraction, is a conse- 

 quence of the unequal retardation experienced by the different waves, 

 which produce the different colors, in their transmission through the 

 refracting medium. This unequal retardation presupposes differences 

 in the velocities with which the various-colored rays are transmitted 

 through any medium whatever, and a dependence of these velocities 

 upon the lengths of the waves. But, according to a well-established 

 mechanical theorem, the velocities with which undulations are prop- 

 agated through a continuous medium depend solely upon the elasticity 

 of the medium as compared with its inertia, and are wholly indepen- 

 dent of the length and form of the waves. The correctness of this the- 

 orem is attested by experience in the case of sound. Sounds of every 

 pitch travel with the same velocity. If it were otherwise, music heard 

 at a distance would evidently become chaotic ; differences of velocity 

 in the propagation of sound would entail a distortion of the rhythm, 

 and, in many cases, a reversal of the order of succession. Now, differ- 

 ences of color are analogous to differences of pitch in sound, both re- 

 ducing themselves to differences of wave-length. The lengths of the 

 waves increase as we descend the scale of sounds from those of a higher 

 to those of a lower pitch ; and similarly, the length of a luminar undu- 

 lation increases as we descend the spectral scale, from violet to red. It 

 follows, then, that the rays of different color, like the sounds of differ- 

 ent pitch, should be propagated with equal velocities, and be equally 

 refracted ; that, therefore, no dispersion of light should take place. 



