-1859] WRITINGS OF JOSEPH HENRY. 103 



tremity. But while this motion takes place through the centre 

 line of the assemblage of atoms, the motion of a will also 

 bring it nearer to the atoms b and c, on either side ; and 

 these will therefore be repelled from their positions of qui- 

 escence, and lateral waves in which the atoms vibrate trans- 

 versely to the direction of the ray, will be produced. It is 

 probable that both kinds of vibration are transmitted through 

 the setherial medium, and perhaps both also through the air ; 

 but such is the constitution of our eyes that they can only 

 perceive the results of those of the second kind, and such the 

 constitution of our ears that they can only take cognizance of 

 those of tlie first. The transverse vibration of light and 

 heat was a happy conception of Dr. Thomas Young, (one of 

 the discoverers of the key to the Egyptian hieroglyphics,) 

 and was applied by himself and Fresnel to the explanation 

 of a large and interesting series of facts classed under the 

 name of polarization of light and heat. 



Besides the invisible emanation from the sun, which gives 

 us the sensation of heat, there are others equally invisible 

 which produce other effects. Indeed it is possible that there 

 are an indefinite number of waves, differing in length and 

 perhaps in form, though many of these must be so minute 

 as to produce no appreciable physical effect at the distance 

 of our planet. If a beam of light be decomposed by a prism, 

 it is well known that it will be separated into parts, pro- 

 ducing different colors. Now if we subject to this spectrum 

 a piece of paper which has been soaked in a solution of 

 nitrate of silver, we shall find that the salt of silver will be 

 decomposed, and the paper will be blackened by the reduced 

 metal. But the interesting part of the experiment is that 

 the blackening will be more intense at a point in the pro- 

 longation of the spectrum, which is entirely in the dark. 

 There is then in a sunbeam, besides light and heat, a ray which 

 may be separated from the former by a prism, which pro- 

 duces chemical decomposition, and is hence called the chem- 

 ical ray. I need scarcely remark that it is this ray, and not 

 that of light, which produces the picture in the photo- 

 graphic and daguerrean processes. 



