LIGHT. 



made use of. If the wick be too large, it 

 will supply a greater quantity of the fluid 

 than can be well decomposed. Its evapo- 

 ration will therefore diminish the tempe- 

 rature, and consequently the light, and 

 afford a fuliginous column, which will 

 pass through the centre of the flame, and 

 fly off in the form of smoke. The mag- 

 nitude of the wick may, from time to 

 time, in candles, be reduced, as to length, 

 by snuffing ; but this operation will not 

 remedy the evils which arise from too 

 great a diameter. If the oil be not suf- 

 ficiently combustible, the ignition will be 

 but moderate, and the flame yellow ; and 

 the same effect will be produced, if the 

 air be not sufficiently pure and abundant. 

 An experiment to this effect may be made 

 by including the flame of a small candle 

 or lamp in a glass tube of about one inch 

 in diameter, standing on the surface of a 

 table. The air which passes between 

 the glass and the table, will be sufficient 

 to maintain a very bright flame ; but if a 

 metallic covering, perforated with a hole 

 of about a quarter of an inch in diameter, 

 be laid upon the upper orifice of the tube, 

 the combustion will be so far impeded, 

 that the flame will be perceptibly yellow- 

 er. The hole may then be more or less 

 closed at pleasure by sliding a small piece 

 of metal, for example a shilling, over it. 

 The consequence will be, that the flame 

 will become more and more yellow, will 

 at length emit smoke, and if the whole be 

 entirely closed, extinction will follow. 



The smell arising from the volatile 

 parts which pass oft' not well consumed 

 from a lamp or candle, must be differ- 

 ent according to the nature of those 

 parts. This depends chiefly on the oil, 

 but in some measure upon the wick. 

 When a candle with a cotton wick is 

 blown out, the smell is considerably 

 more offensive, than if the wick be of 

 linen, or of rush ; but less offensive than 

 if the supply of the combustion had 

 been oil. Whenever a candle or lamp 

 is removed, the combustion is in some 

 measure impeded by the stream of cold 

 air, against which it strikes. Smoke is 

 accordingly emitted from its anterior 

 side, and the peculiar smell is perceiv- 

 ed. From this imperfection, lamps are 

 much less adapted to be carried from 

 place to place than candles. 



From the necessity of the access of 

 air, there will be mare light produced 

 from a lamp with a number of small 

 wicks, than with one large one, or from 

 a number of small candles, than the 

 same quantity of tatfow used to make 



a single large one. In the lamp of Ar- 

 gand, the wick consists of a web of 

 cloth in the form of a pipe or tube, the 

 longitudinal fibres of which are thicker 

 than the circular ones. This is passed 

 by a suitable contrivance into a cylin- 

 drical cavity, which contains the oil; 

 and there are other precautions in the con- 

 struction of the apparatus, by which the 

 oil is regularly supplied, the access of 

 air is duly permitted, as well within as 

 without the circle formed by the upper 

 edge of this cylindrical wick, and ihis 

 edge can be raised or lowered at plea- 

 sure. Hence the possessor has it in his 

 power to regulate the surface of the 

 wick, so that the greatest flame consist- 

 ent with perfect combustion may be 

 produced ; and the steadiness of the 

 flame is secured by a glass shade or 

 tube, which surrounds it, and in a cer- 

 tain degree accelerates the current of air. 



In the illumination by candles, where 

 the fused matter is contained in a cup or 

 cavity of the matter not yet fused, it is of 

 some consequence, whether the substance 

 be fusible at a high or low temperature. 

 The difference between wax and tallow 

 candles arises from this property. Wax 

 being less fusible, will admit of a thinner 

 wick, and needs no snuffing ; but in a tal- 

 low candle it is absolutely necessary to 

 have a large wick, capable of taking up 

 the tallow as it melts. 



The difference of effect in illumination 

 between a thick and a thin wick cannot 

 be better shown, than by remarking the 

 appearances produced by both. When a 

 candle with a thick wick is first lighted, 

 and the wick snuffed short, the flame is 

 perfect and luminous, unless its diameter 

 be very great ; in which last case, there 

 is an opaque part in the middle, where 

 the combustion is impeded for want of 

 air. As the wick becomes longer, the 

 space between its upper extremity and 

 the apex of the flame is diminished ; and, 

 consequently, the oil, which issues from 

 that extremity, having a less space of 

 ignition to pass through, is less complete- 

 ly burned, and passes offpartly in smoke. 

 This evil continues to increase, until at 

 length the upper extremity of the wick 

 projects beyond the flame, and forms a 

 support for an accumulation of soot, which 

 is afforded by the imperfect combustion. 

 A candle in this situation affords scarcely 

 one-tenth of the light, which the due com- 

 bustion of its materials would produce ; 

 and tallow candles, on this account, re- 

 quire continual snuffing. But, OH the 

 contrary, if we consider the wax candle. 



