53 MELLONI'S EXPERIMENT OF LIGHT WITHOUT HEAT. 



when we attempt similar explanations for other facts. Thus, a thin plate of transparent 

 alum and a thick mass of smoky rock crystal, the former completely diaphanous, the lat- 

 ter so opaque that even when placed in the fullest light it was not possihle to read large 

 printed characters, being compared together, the latter was found to be more transparent 

 to heat than the former in the proportion of 19 to 6; and, indeed, if these agents be iden- 

 tical, and the rays of light have a proper heat of their own, so that their amount may 

 be measured by a thermometer, what becomes of such experiments as the following, 

 brought forward, on the other side of the question, by M. MELLOW, formerly, who pass- 

 ed a ray of light through a stratum of water, and then through a glass tinged green by 

 the oxide of copper? " The pure light emerging from this system contains much yel- 

 low, and possesses, at the same time, a tinge of bluish green. It exhibits no calorific 

 action capable of being rendered perceptible by the most delicate thermoscopes, even when 

 it is so concentrated by lenses as to rival the direct rays of the sun in brilliancy." ( Tay- 

 lor's Scientific Memoirs, vol. i., p. 392. 



205. Until, therefore, an explanation can be given of this experiment, or its authenticity 

 disproved, the recognised rules of chemistry require that we should speak of light and 

 heat as distinct agents. Nor are these the only imponderable principles which are in- 

 volved ; there are others known to exist, the position of which is determined by these 

 discussions : the chemical rays, for example, which M. BECQUEREL regards as nothing 

 more than invisible light, adopting for them the same theory which M. MELLONI has 

 adopted for the rays of heat. There are also rays which can excite the glow from phos- 

 phorescent bodies. From the time of FRANKLIN and CANTON it has been known, that 

 if calcined oyster shells were exposed to the rays of an electric spark, as, for example, 

 to the discharge of a Leyden vial, the shells would commence to shine ; but, as may 

 easily be proved, it has more recently been found, that if the rays thus exciting the 

 phosphorescent quality be transmitted through a piece of colourless flint glass, they 

 can no longer produce the result. It might be said that this arises from the fact that 

 colourless glass has an invisible coloration, and that it absorbs the rays which are far 

 beyond the violet, but direct experiment proves that flint glass possesses no such quality. 



206. Inasmuch, therefore, as a piece of flint glass can cut off those dark rays which 

 excite phosphorescence in the sulphuret of lime, and combinations such as that which 

 has been referred to (204) are known, which can separate light from heat, we are jus- 

 tified in regarding these as distinct imponderable principles. They may, it is true, be 

 remotely identical, but they are not identical in the way here set forth. They may 

 all consist of similar movements or undulations of one primordial ether, but there are 

 points in which the mechanism of those undulations vary circumstances which im- 

 press upon them physical differences. As, in the phenomena of sound, we may have 

 instruments which give rise to undulations of the same length and the same intensity 

 instruments which are executing the same strain of music at the same time yet we 

 know well enough that each one impresses its own proper modifications, which the ear 

 in an instant detects. At a distance, we recognise the flute, the violin, the piano, the 

 bugle, from one an :t' er. 



207. The imperfections of the prismatic spectrum, which have been just described 



