SOURCES OF LIGHT. 1.23 



The cultivators of modern science are a bold race ; not 

 contented with endeavouring to understand the physical 

 earth, they are endeavouring to comprehend the condi- 

 tion of the solar surface. The mind of man can pene- 

 trate far into nature, and, as it were, feel out the mys- 

 teries of untraversed space. The astronomer learns of 

 a peculiar condition of light, which is termed polarisa- 

 tion, and he learns by this, too, that he can determine if 

 from a bright luminous disc the light is derived from a 

 solid mass in a state of intense ignition, or from vapour 

 in an incandescent condition. He adds a polarising ap- 

 paratus to his telescopes, and he determines that the 

 light we derive from the sun is due to an envelope of 

 vapour burning, in all probability only with greater 

 intensity, as the gas which we now employ. This Pho- 

 tosphere as it has been called by the late French philo- 

 sopher Arago, is found to be subjected to violent disturb- 

 ances, and the dark spots seen on the sun's disc are now 

 known to be openings through this mysterious envelope 

 .of light, which enable us to look in upon the dark body 

 of the sun itself. 



Luminous phenomena may be produced by various 

 means chemical action is a source of light ; and, under 

 several circumstances in which the laws of affinity are 

 strongly exerted, a very intense luminous effect is pro- 

 duced. Under this head all the phenomena of combus- 

 tion are included. In the electric spark we have the 

 development of light ; and the arc which is formed 

 between charcoal points a the poles of a powerful voltaic 

 battery affords us the most intense artificial illumination 

 with which we are acquainted. In addition to these, we 

 have the peculiar phenomena of phosphorescence arising 

 from chemical, calorific, electrical, actinic, and vital ex- 

 citation, all of which must be particularly examined. 



From whatever source we procure light, it is the same 

 in character, differing only in intensity. In its action. 



