156 Management of Light in Illumination. 



will be too long to be extinguished suddenly, and with- 

 out smoke, after having been lighted for the first time. 

 If attention be paid to it, no disagreeable smell whatever 

 will be diffused on that occasion, nor on any other. 



All the lamps with which I am acquainted diffuse a 

 very noxious, stinking vapour when they are made to 

 burn with a very small flame. Even an Argand lamp, 

 in which the combustion of the oil is usually so com- 

 plete, if it be so arranged by lowering its wick as to 

 give only about one sixth part of the light it usually 

 furnishes, it will diffuse a smell so very offensive that 

 it will become quite insupportable. 



To see clearly into this matter, we have only to 

 consider what the changes are which take place when 

 an Argand lamp, burning with its usual vivacity, is 

 suddenly made to burn with a very feeble flame. 



When this lamp burns well, the current of air which 

 passes upwards through its chimney is so strong that 

 the flame of the lamp is forced upwards towards the 

 upper end of the wick ; and the burner, being at some 

 distance from the flame, is kept so cool by this strong 

 blast of cold air that it does not become sufficiently hot 

 to decompose the oil with which it is alway in contact ; 

 but, as soon as the wick is considerably shortened, the 

 flame being much diminished, the current of air through 

 the chimney becomes very feeble, and the flame, being 

 no longer forced upwards by that current, descends by 

 degrees, till at last it establishes itself on the very brim 

 of the burner. This necessarily heats the top of the 

 burner very hot, however small the flame may be ; and, 

 as all the oils which are used in lamps are decomposed 

 and evaporated at a lower temperature than that at 

 which they take fire and burn, the cause of the offen- 



