164 Physical and Chemical Properties of ike [Seit- 



rnatter of the prisms and the difference of the apparatus, or on 

 some circumstances depending upon the phenomenon itself? It 

 is not possible for us to determine. 



M- Berard wished to know if these properties would exist in 

 each of the pencils into which the ray divides itself in passing 

 through a rhomboid of Iceland spar. He made a ray of solar 

 light pass through a prism formed of a piece of Iceland spar. 

 Each of the two spectx-a exhibited the same properties. In both, 

 the calorific power diminished from the violet to the red end, and 

 it existed beyond the last visible red rays. Thus, whether this 

 faculty be inherent in the solar rays or not, when these rays are 

 divided by a crystal it goes along with each. 



But in this operation the luminous molecules are polarized by 

 the crystal. Do the invisible rays of heat experience the same 

 effect ? To determine this point, M. Berard received the solar 

 ray upon a polished and transparent glass, which polarized a 

 portion of it by reflection. This reflected ray was then received 

 upon a second glass, fixed in an apparatus, which permitted it to 

 be turned round the ray under a constant incidence, and this 

 incidence itself was determined in such a manner that in a cer- 

 tain position of the glass the reflection ceased to take place. We 

 know, from the experiments of Malus, that a glass may be 

 always disposed in such a manner that this condition is fulfilled. 

 Things being thus disposed, by collecting with a mirror the 

 calorific and luminous rays reflected from the second glass, and 

 directing them upon a thermometer, M. Berard found, that as 

 long as light was reflected the thermometer was elevated, and of 

 course the heat was reflected likewise; but when, from the posi- 

 tion of the second glass, the light was totally transmitted, the 

 heat was transmitted at the same time, and the thermometer was 

 not elevated. In this experiment, then, as well as the preceding, 

 the calorific principle, whatever it may be, never separates from 

 the luminous molecules. 



To the ray of solar light employed in this experiment, M. 

 Berard substituted a pencil of radiant heat proceeding from a 

 body hot, but not red, and even not luminous. The effect was 

 the same as before. The thermometer rose when the second 

 glass was so situated as to reflect light, and it did not rise when 

 the second could not reflect light. Therefore the particles of 

 invisible radiant heat are modified by reflection, precisely like 

 light. 



After having studied the calorific properties of the different 

 rays of the spectrum, M. Berard examined their chemical pro- 

 perties. When muriate of silver, or other white salts of silver, 

 are exposed to light, they become dark coloured very speedily. 

 Guaiacthus exposed to light passes from yellow to green, as Dr. 

 Wollaston observed. Gay-Lussac and Thenard discovered 



