210 WELLS'S NATURAL PHILOSOPHY. 



by the vibrations of an ethereal fluid occupying all space, supposes that the 

 sun may produce the phenomena of light and heat without waste of its tem- 

 perature or substance, as a bell may constantly produce the phenomena of 

 sound. 



Whatever may be the true theory, a series of experiments, made some years 

 since by Arago, the eminent French astronomer, seem to prove that the tem- 

 perature at the surface of the sun is much more elevated than any artificial 

 heat we are able to produce. The experimental reasons which lead to this 

 opinion are as follows : — 



There are two states in which light is capable of existing — the ordinarf 

 state, and the state of polarization.* It has been proved that all bodies, in i 

 solid or liquid state, which are rendered incandescent by heat, emit a polar- 

 ized light, while bodies that are gaseous, when rendered incandescent, inva- 

 riably emit Hght in its ordinary state. Thus the physical condition of a body 

 may be distinguished when it is incandescent by examining the light which 

 it affords. On applying the test to the direct light of the sun, it was found to 

 be in the unpolarized or ordinary condition of Ught. Hence it has been in- 

 ferred by Arago that the matter from which this light proceeds must be in 

 the gaseous state, or, in other words, in a state of flame. From other experi- 

 ments and observations, Arago was led to the conclusion that the sun was a 

 solid, opaque, non-luminous body, invested with an ocean of flame. 



_. . ^ , 471. Ovvinoj to the position of the earth's 

 ative heat of axis, the relativ^c amount of heat received from 



the sun always i • i • • /> 



greater in some the suii IS alwavs greater in some portions of 



portions of the , i i i • ^ n t 



earth than at the oarth than at others, smce the rays oi the 



others? , />n ti i i 



sun always lall more directly upon the central 

 portions of the earth than they do at the poles, or extremi- 

 ties ; and the greatest amount of heat is experienced from 

 the rays of the sun when they fall most perpendicularly. 



Why is the ^'^'^' "^'^^ '^^^^ ^^ ^^^® ^^'^ ^^ greatest at noon, because for 



heat of the sun the day the sun has reached the highest point in the heavens, 

 Boon^r' *' and its rays fall more perpendicularly than at any other 

 time. 



What occasions for a Uko reason we experience the extremes of tempera- 



the diiference . j. ■ . 



in temperature ture, distinguished as summer and winter. In summer tlie 



win"erT''^ ^""^ position of the sun in relation to the earth is such, that al- 

 though more remote from the earth than in winter, its rj-fj-a 

 fall more perpendicularly than at any other season, and impart the greatest 

 amount of heat ; while in winter the position of the sun is such that its rays 

 fall more obliquely than at any other time, and impart the smallest amount of 

 heat. The sun, moreover, is longer above the liorizon in summer than in 

 winter, which also produces a corresponding effect. 



The reason why a difference in the inelination of the sun's rays falling upon 

 • For explanation of the term polarization, see chapter on Light 



