Effects of Heat and Light on Inorganic Bodies. 409 



CH AFTER XLIL 



EFFECTS OF HEAT AND LIGHT ON INORGANIC BODIES. 



It is a principle of plrysics that action and reaction are equal. There- 

 fore, we must hold that whenever a form of energy is altered by trans- 

 fer to and collision with a new body, the amount and value of the alter- 

 ation is to be found in changes in the body itself. In other words, the 

 body cannot change the form of the energy without suffering an equal 

 change from its own former condition. 



Having seen some of the changes effected in light by its encounter 

 with ponderable bodies, we are now prepared for some illustrations of 

 the way they are affected by it. We have already met with numerous 

 examples of the effects of heat and light in the study of other topics. 

 Thus the action of heat is essential in preparing the necessary condi- 

 tions of temperature for all sorts of chemical reactions and all the phe- 

 nomena of organic existence. And so is that of light. The service of 

 light in connection with plant growth and the formation of starch, has 

 been mentioned. The influence of these agents in bringing about the 

 several allotropic states to which a number of the so-called original ele- 

 ments are liable, has also been mentioned. Thus heat at 290 destroys 

 the molecule of ozone and enables the atoms to re-form themselves into 

 oxygen. And either light or heat, under certain conditions, may con- 

 vert oxygen into ozone. 



Selenium, arsenic, sulphur, phosphorus and carbon are all changed 

 by heat or light into their various allotropic states. Amorphous selen- 

 ium is a non-conductor of electricity, but it is changed by heat into its 

 crystalline form, in which it is a conductor. Its conductivity is much 

 greater in the light than in darkness. In an experiment made by Wil- 

 loughby Smith, the conductivity increased from 15 per cent, to 100 per 

 cent, under the influence of the light of a common gas burner. It has 

 been found that the resistance in selenium is in direct ratio to the 

 square root qf the illuminating power. But the most remarkable fact is 

 that light shining on a piece of selenium is instantly converted info an 

 electrical current. This current starts up and stops as rapidly as light 

 <-:in be turned on and off. 



Chlorine also, as mentioned, may exist in two states, in one of which 

 it is active, in the other passive. When it is prepared in the dark it is 

 passive ; and in this condition it may be mixed in a glass jar with an 

 equal ^volume of hydrogen without a chemical union between the two 

 taking place. But if the mixture be exposed to sunlight, the chlorine 

 becomes " active," and hydrochloric acid is formed by the union of the 

 two, with an explosion which may break the jar unless it be strong. 



