LIGHT. 33 



These can be regarded as only general truths subject of course to 

 many exceptions and qualifications. 



In birds, the parts exposed to the light, as the back and breast, are 

 always colored, but the feathers beneath the wings and under the 

 belly are usually white. So the back and fins of fishes are colored, 

 while the belly is white. Snakes and other reptiles, and the am- 

 phibious animals are distinguished in the same manner. 



(c.) The color of the human species is generally graduated in tol- 

 erable accordance with the quantity of light ; black people are not found 

 perhaps any where except within the tropics, and white ones no where 

 but in the northern temperate zone ; but the state of society, food, 

 habitations, employments, and many other causes, modify these results, 

 and it is to be observed, that the colored people of polar climates, are 

 all barbarians, living in smoke, filth, exposure and wretchedness.* 



(d.) The color of persons, of the various classes and conditions 

 of society, accords with this view. Students and artisans, working 

 within doors, and women, whose employments are, in this coun- 

 try, generally in the house, are of lighter complexions ; while far- 

 mers, sailors, soldiers, &c. are more deeply colored. Many other 

 causes, especially those affecting the state of health, do however mod- 

 ify these results. 



(e.) Light is necessary to health and cheerfulness. Animals and 

 men, confined in darkness, become gloomy, and their health and their 

 faculties are gradually impaired. 



In the human subject, when long deprived of light, dropsy is said 

 often to terminate life. 



Other physical causes also operate, as want of exercise and bad 

 air, and moral causes must also powerfully affect the human mind. 



Even animals are affected, in a way somewhat analogous. 



18. LIGHT ACTS ON MINERAL BODIES. 



JVitric acid is decomposed into nitrous acid and oxygen gas. 



Aqueous solution of chlorine gives out oxygen and muriatic acid, 

 and most rapidly in the most refrangible rays. 



Metallic oxides are, in some instances, decomposed ; the oxides of 

 mercury sometimes give running mercury. 



fVhite muriate of silver becomes dark, and even black, muriatic 

 acid gas being formed. 



Chlorine and hydrogen gases, in equal volumes, explode by the 

 stroke of the solar ray and very quickly in the violet ray. 



Phosphorus which is white, when first distilled in hydrogen gas, 

 becomes colored, yellow and brown, by the action of light. 



Substances wet with nitrate of silver, become dark, and even black, 

 by exposure to the sun. 



* See Dr. S. S. Smith's Essay ; also the learned work of Dr. Pritchard, on the 

 physical history of man. 



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