126 Introduction to the Study of Science 



the test tube, and when cool heat it directly to a high temperature. 

 It will burn completely if charcoal ; or if coke, all but a bit of ash will 

 burn, for both are practically pure carbon. How can you prove that 

 they are carbon? Does this show that smoke consists largely of un- 

 burned substances which will burn in proper conditions ? 



Heat a small amount of soot directly in a flame, as you did the 

 carbon. Does it burn when heated to a high temperature? If it 

 does, and produces carbon dioxid, as you may determine by the 

 limewater test, what do you infer it to be? Does it behave as char- 

 coal and coke in similar conditions? 



In combustion carbon particles of very minute size combine 

 at any temperature which is too low to effect complete combus- 

 tion. Thus are formed visible masses of unburned carbon 

 particles which remain suspended in the invisible gases evolved 

 in imperfect combustion. The gases are combustible as your 

 experiments show, and burn to water vapor and carbon dioxid. 

 They are usually made up of hydrogen and carbon monoxid, 

 which gases form what is known as water gas (pages 162-3). 

 These facts prove conclusively that smoke with its visible 

 soot and invisible gases is not a useless material to be driven 

 off and lost. 



Make tests of other fuels for carbon by the same method followed 

 in collecting smoke from the candle. You might test, among others, 

 kerosene, alcohol, and illuminating gas. If any of these fails to show 

 the presence of carbon by forming soot, its products of combustion 

 may be tested for carbon dioxid. 



In the use of coal or other carbon compound for the pro- 

 duction of heat, it is possible to allow the carbon to be wasted in 

 two ways : (1) it may be driven away in fine particles, forming 

 a black smoke ; (2) it may unite with only half the amount of 

 oxygen necessary for complete combustion, and the incompletely 

 burned carbon or carbon monoxid escape as an invisible gas. 



It is well established that one pound of carbon produces, if 

 burned completely, 14,544 British Thermal Units (page 142). 

 It has been proven also 'that one pound of carbon produces 

 about 5000 B. T. U., when it is only partially burned or changed 



