COMBUSTION. 23 



and sodium* when burnt is an alkali ; but alkalies are 

 now considered metallic oxides. 



The nature of combustion will be rendered further 

 intelligible by simply considering the burning of a com- 

 mon candle. In lighting a candle, which must be done 

 by the application of flame, a small portion of the wick 

 is first decomposed or "burnt, the tallow becomes gra- 

 dually melted, ascends the wick by capillary attraction, 

 is decomposed, and exhibits itself in flame. The com- 

 ponent parts of tallow are hydrogen and carbon ; the 

 hydrogen unites with a portion of the oxygen, and forms 

 water, which is carried off in vapour ; and the carbon, 

 when the combustion is perfect, unites with another 

 portion of the oxygen, and is carried off in a gaseous 

 state. In common tallow candles the top of the wick is 

 always black, from the flame preventing the air from 

 oxidizing it, and forms a support, if the candle be not 

 snuffed, for an accumulation of carbon, which is pro- 

 duced through the imperfect combustion. la wax 

 candles, and also in Palmer's patent metallic-wicked 

 candles, the wick is made so as to lean aside out of the 

 flame ; the combustion is then complete, and the 

 wick neither collects soot, nor requires to be snuffed. t 



oxygen of the water with such rapidity as to decompose it and ignite 

 the hydrogen. This is a very interesting experiment, as the potassium 

 swims about on the surface of the water, apparently burning with a 

 beautiful bright flame. Potassium is the basis of potash, the vegetable 

 alkali, which latter is properly an oxide of potassium, and together 

 with its salts is of much use in the arts, medicine. 6cc. Potash is 

 formed from the ashes of burnt wood by lixiviation ; it is also pre- 

 pared in wine countries from the wine lees. 



* Sodium is also considered a metal, and although heavier than 

 potash it is lighter than water. It has a great affinity for oxygen, and 

 will decompose water, yet not with that violence as to produce flame. 

 Sodium is the base of the mineral alkali Soda, which, like potash, to- 

 gether with its salts, is very much used in the arts, medicine, &c. 

 The two alkalies potash and soda, particularly the latter, are indis- 

 pensable to the soap-maker, the glass-maker, and the dyer. Soda is 

 obtained from sea-weed, and also from salt, which is a chloride of 

 sodium ; it is also found in great plenty combined with various sub- 

 stances. 



f It" a common candle be inclined more than 30 from a perpendi- 

 cular, it \vill not require to be snuffed. 



