LIGHTING. 



air is constantly supplied through the bars directly 

 to the fuel while burning, and in this way perfec 

 combustion is obtained. The bars, being slowly 

 moved on, carry the ashes to the ashpit, which 

 lies at the back of the grate. Jukes's apparatus 

 was applied to the furnace of the engine which 

 prints this work in 1848, and has been completely 

 successful ; it is rare that a single particle ol 

 smoke can be seen issuing from the chimney, and 

 the saving in coal and attendance is decided. 



LIGHTING. 



Artificial lighting depends upon the fact that 

 solid bodies, when heated to a certain degree, 

 become luminous or incandescent. The luminosity 

 begins at the temperature of about 800 ; and as 

 the heat is increased, the red passes into a white 

 of greater and greater brilliancy. 



Gaseous bodies, however hot, give little light. 

 In order to produce light, we must have a highly 

 heated solid substance. In practice, this is best 

 obtained by burning volatile compounds of carbon 

 and hydrogen. The combustion of the hydrogen 

 produces intense heat, and the particles of solid 

 carbon, before being themselves burnt, become 

 white in the hot gas. 



While pure carbon may be used as fuel for pro- 

 ducing heat, carbon and hydrogen united, and 

 burning in the shape of flajne, produce most light, 

 because the combustion is more rapid, and the 

 heat more intense. Carbon and hydrogen com- 

 bine in a variety of ways, and produce a numerous 

 class of compounds, called hydro-carbons, some of 

 which are solid, some liquid, 

 and others gaseous, but mostly 

 capable of being vaporised by 

 heat. A number of natural 

 products, such as the animal 

 and vegetable oils, tallow, wax, 

 &c are altogether composed 

 of such compounds, and can 

 be vaporised or distilled with- 

 out leaving any solid residue. 

 These form the readiest 

 sources of light, because the 

 substances may be distilled or 

 vaporised by the heat of the 

 flame that produces the light, 

 as in the case of a candle. 



Structure of Flame. The 

 flame of a lamp or candle, or simple gas-jet, 

 consists of a hollow cone, in the centre of which 

 there is no combustion. 



The central space appears dark only by contrast 

 with the luminous cone which surrounds it. It 

 consists, in reality, of transparent invisible com- 

 pounds of carbon and hydrogen, which are con- 

 stantly rising in vapour from the wick. If a glass 

 tube, open at both ends, be held obliquely in the 

 flame of a candle, with its lower extremity in the 

 dark central space above the wick, it will conduct 

 away a portion of the combustible vapour, which 

 may be kindled like a gas-jet at its upper end, as 

 represented in fig. 9. This dark portion of the 

 flame may be called the area of no combustion. 



The luminous cone which envelops the dark 

 space is the area of partial combustion. The 

 oxygen of the atmosphere penetrates to this 



Fig. 9. 



Fig. i a 



depth, but not in sufficient quantity to oxidise or 

 burn both the carbon and the 

 hydrogen ; it therefore unites 

 with the hydrogen, for which it 

 has the stronger attraction, and 

 leaves the carbon free. The 

 outer cone is named the area 

 of complete combustion, because 

 there the carbon meets with 

 sufficient oxygen to burn it en- 

 tirely. The light is produced in 

 the area of partial combustion, 

 where the carbon is set free . . 8 . .. 

 from the hydrogen in the form , Area of no com. 

 of solid particles, and is heated bustion. 

 to whiteness by the combustion ' ^ist'ST^ C m ~ 

 of the hydrogen. The combus- c, Area of complete 

 tion of the carbon in the outer combustion, 

 cone, by which it is converted into carbonic acid 

 gas, produces heat, but so little light as to be 

 barely traceable. 



That carbon exists in a solid state in the white 

 part of a flame, is readily shewn by holding a piece 

 of white earthenware into it, which becomes coated 

 with carbon in the form of soot. 



The highly illuminating power of compounds of 

 hydrogen and carbon is thus traced to the fact, that 

 their hydrogen and carbon do not burn simultane- 

 ously, but successively, and in such a way that the 

 one heats the other white-hot. It is quite possible 

 to make them burn simultaneously ; but when they 

 do, the light evolved is very feeble. This is seen 

 in the Bunsen burner alluded to, p. 485. 



ILLUMINATION BY CANDLES AND LAMPS. 



In principle, these two modes of lighting are 

 the same. The essential parts of a lamp are a 

 vessel containing liquid fat, from which a portion 

 rises gradually by capillary attraction through the 

 wick to the flame. In a candle, the solid fat below 

 the flame is melted into the form of a hollow cup, 

 which forms a reservoir for the liquefied portion, 

 and becomes thus a tallow lamp. 



The chief difficulty that attends the use of lamps 

 as a source of light is, to procure the complete 

 combustion of the oil, so as to keep the flame from 

 smoking. The round cotton-wick used in the old 

 simple form of lamp was always attended with 

 smoke and smell. The oils and fats are exceed- 

 ngly rich in carbon, containing 70-80 per cent, of 

 hat element, and only 10-12 of hydrogen. The 

 round thick column, then, of oil-vapour rising from 

 :he wick of an old-fashioned lamp, presents too 

 ittle extent of surface to the air ; the oxygen of all 

 he air that can get access is chiefly taken up in 

 jurning the hydrogen, and a large proportion of 

 he carbon ascends in the burnt air as smoke. 

 The most essential improvement in this respect is 

 what is known as the Argand burner, from the 

 name of its inventor. In this the wick is in the 

 brm of a ring. The flame is thus a hollow cylin- 

 der, with a current of air ascending through the 

 nside, so that the burning surface is doubled. 



Another part of Argand s improvement consisted 

 n placing a glass cylinder as a chimney over the 

 lame, by which means the flame is steadied and a 

 draught created. 



Subsequent improvements have had for their 

 object to deflect the currents of air from their 

 parallel course, and make them strike against the 



451 



