124 Transactions of the American Institute. 



Such, then, is, in general, the character of the light-force ; but 

 fully to appreciate its nature we must look somewhat at its relations 

 to the other natural forces. 



The light-force belongs to a small group which may be defined as 

 " modes of motion," and also, in a certain sense, as " the known 

 forces." This group includes, beside light, heat and sound. 



We know as well as we know anything in science, and this is not 

 saying a little, that all these three forces are modes of motion, and 

 indeed, more definitely still, vibratory modes of motion, and yet 

 more precisely, modes of vibratoiy motion, one of whose marked 

 distinctions is, that in the case of sound their recurrence is limited 

 between the bounding terms of thirty-two vibrations, and some 

 10,000 in a second; in the case of heat, between 200,000,000,000,000 

 and 400,000,000,000,000 in a second, and in the case of light, 

 between 400,000,000,000,000 and 800,000,000,000,000 in a second. 



This is very much more than we can say of any of the other forces 

 of nature, such as gravitation, adhesion, repulsion, electricity, etc., 

 for we do not know the very first thing as to their nature, proximate 

 or remote, and cannot so much as venture an opinion as to whether 

 any of them are modes of motion or not. 



We see, therefore, that there is some propriety in calling Sound, 

 Heat and Light known forces, since our knowledge as to their nature 

 reaches so far, as compared with its extent in other like directions. 



But we must not be betrayed into regarding our knowledge even 

 in this case as absolute. This vibratory motion depends for its 

 maintenance and propagation upon certain properties of density, 

 elasticity, etc., in the Luminiferous Ether, in the Atmosphere and in 

 other materials, but these properties of density and elasticity are sim- 

 ply the expressions of Cohesion, Polarity, and other of the unknown 

 forces, so that, if we push our study on to the extent of " final 

 causes," we find ourselves here, as elsewhere, gazing over that vast 

 ocean of infinite power and creative force, from which the billows of 

 exhaustless energy are ever rolling in at our feet, but never bringing 

 to us any evidence of the nature of their origin other than that of its 

 omnipotence and divine beneficence. 



Sound, heat and light, then, are all modes of vibratory motion, 

 whose principal distinction we may regard as simply that of rapidity. 



And here I may well mention a most suggestive idea thrown out 



Note. — I considered it inexpedient to enter into the question of the distinctive character between 

 sound and light, and heat vibrations, involved in the longitudinal character of the first, and the 

 transverse nature of the others, especially as this was to be fully treated in a succeeding lecture by 

 Prof. Rood. 



