LIGHT. 31 



Combustion is a familiar and very general example. Phos- 

 phorescence, without combustion, is seen in the case of sulphuric acid 

 and calcined magnesia ; when the magnesia has been recently and 

 thoroughly calcined and the sulphuric acid is strong, there is almost 

 always (especially if a few ounces of the materials be used) a flash 

 in the dark, and sometimes it is visible in the day light. 



Lime slaking in the dark, sometimes shows luminous points. 

 Light is emitted during the combination of sulphur and metallic 

 filings, as copper and iron of potassium and sulphur, iodine and phos- 

 phorus, &c. ; that from iodine and phosphorus is very vivid. It is ne- 

 cessary only to throw a lijtle iodine upon a small piece of phosphorus 

 in a dry wine glass ; the action is speedy or even instantaneous ; a mild 

 heat may bring it on when it is tardy, but we should be on our guard 

 against explosion. The same remarks will apply to iodine and po- 

 tassium, only the action is more violent, and the burning potassium is 

 often thrown about the room. 



15. LIGHT is A CHEMICAL AGENT. 



(a.) It acts on vegetables. Etiolation or bleaching of vegetables 

 by tying them up, takes effect in consequence of the exclusion of 

 light. Celery is white, mild, and agreeable when growing beneath 

 the earth, but acrid, and as is said, even poisonous if growing in the 

 light ; the potatoe root is affected in a similar manner by light. 

 Shoots of potatoes, turnips, cabbage, parsnip, carrot, fyc. are mild 

 and white when sprouting in a moist, dark cellar, but if a beam of 

 light crosses them, as from a crack, or a hole in a window, they be- 

 come colored and pungent, and incline towards the light. The in- 

 side leaves of heads of cabbage or lettuce are white and tender ; so 

 are the inner coats of onions, the bottom parts of blades of grass, 

 especially when shooting from beneath a flat stone, and vines when 

 growing in the same manner. 



The bark of trees is generally more colored than the wood but 

 ivoods are occasionally deep colored, as the dye woods and roots, 

 logwood, fustic, brazil-wood, lignum vitae, madder, turmeric, quercit- 

 ron, alkanet, &,c. and the heart of wood is sometimes more deeply 

 colored than the superior layers, as in the red walnut. 



Many causes operate besides light, e. g. heat, air, chemical com- 

 position, &tc. But even colored woods generally grow deeper color- 

 ed by exposure to light, e. g. mahogany, cherry, black walnut, ma- 

 ple, &c. as seen in common furniture. Red roses made to grow in 

 the dark become white, or rather the trees that produce red roses in 

 the light, produce white ones in the dark.* Davy. 



?^.) Although the color of vegetables is not produced exclusively 

 ^ ight, it is owing principally to that cause. 



* Many shells possess rich and varied colors, which from their original situation 

 never have enjoyed direct access to light. 



