LIGHT. 25 



bustion. Inflammation is when the combustion takes 

 place while the combustible is in an aeriform state, as 

 the flame of a candle. Ignition is when the combustible 

 remains in a solid state, as charcoal. Detonation, or 

 Explosion, is caused by the combustible being suddenly 

 converted into a gaseous state, and thereby giving a 

 violent impulse to the surrounding air. 



LIGHT. 



Action of Light on the Animal and Vegetable Kingdoms Nature of Light Its 

 Velocity Direction Refraction Reflection ITie Eye described Vision 

 explained Colours, &c. 



LIGHT consists either of small particles emanating 

 from a luminous body, or it is an excitement produced 

 by undulations or vibrations of a highly elastic medium, 

 filling all space. Although the sun is the chief fountain 

 or cause of light, it is produced in various ways inde- 

 pendent of that luminary. Combustion produces light,* 

 and so does the friction of various substances ; it is also 

 emitted from phosphori of different kinds. 



Light is one of the most useful agents in nature, and 

 one that particularly affects the animal and vegetable 

 kingdoms. The action of light on vegetables causes 

 them to give out oxygen gas, by which means the air 

 becomes purified, and the vital principle restored, which 

 had been vitiated by respiration or other ways. Vege- 

 tables depend on light for their colour : if a vegetable 

 grow in the dark it will be white and sickly, or etiolated, 

 as it is termed. Gardeners blanch certain vegetables by 

 depriving them of light ; and such parts of vegetables as 

 are shaded from the light, as the hearts of lettuces, of 

 cabbages, &c., are generally white. Vegetables in tro- 

 pical regions flourish in a manner far superior to what 

 they do with us ; and discous vegetables, as the sun- 

 flower, always turn towards the sun. 



* A piece of lime of the size of a pea, acted on by an ignited stream 

 of the mixed gases, oxygen and hydrogen, will produce a light equal 

 to 120 wax candles, and with a reflector will cast a shadow ten miles 

 distant. The most intense lighti^that produced by galvanism in an 

 exhausted receiver. 



c 



