1851] WRITINGS OF JOSEPH HENRY. 301 



emission. In order however that this theory may enable 

 us to discover the greatest number of new phenomena, and 

 assist us in ascertaining the more precise relations of known 

 facts, it is necessary that all its parts should be definitely 

 conditioned with reference to established mechanical prin- 

 ciples. The phenomena of light and heat, and of chemical 

 and phosphorogenic emanation from the sun, by strict anal- 

 ogy lead us to infer that something possessing inertia, and 

 obedience generally to the laws of force and motion, must 

 exist between us and this luminary. All the phenomena are 

 best explained and predicted by supposing this something 

 to consist of an elastic medium, the atoms of which, in a nor- 

 mal state, are distributed uniformly through space and 

 retained in position by attracting and repelling forces. An 

 setherial medium, constituted in this manner, will admit of 

 vibrations of different characters and of different forms; for 

 example, if an impulse be given to an atom in a given direc- 

 tion, it will cause in succession a motion to be transmitted 

 to the series of atoms which are found in the same line, and 

 thus longitudinal undulations will be produced; also the 

 motion of the atom to which the impulse is given will cause 

 it to approach the atoms of the medium on the sides of the 

 line just mentioned, and thus rows of atoms on all sides of 

 the first row will be thrown into a state of transverse vibra- 

 tion. Similar systems of vibrations must also take place in 

 air; but such is the constitution of the human ear that it 

 takes cognizance only of longitudinal vibrations, and such 

 the function of the human eye that it is only affected by 

 transverse undulations. Besides these there may be other 

 vibrations compound of the two; and in this way other ema- 

 nations than those which have yet been observed may be 

 conceived to exist. 



The science of electricity, as left by Cavendish and ^pinus, 

 and as expounded by Hauy and Robison, was (next to astron- 

 omy) one of the most perfect of the physical sciences. All 

 the known phenomena of statical electricity were referred to 

 the mechanical action of two species of matter; the atoms of 

 each being self-repellant and attractive of the atoms of the 



