CHEMICAL HISTORY OF A CANDLE 177 



circumstances. Now, what is the circumstance which makes 

 the lead and carbon differ in this respect? It is a striking 

 thing to see that the matter which is appointed to serve the 

 purpose of fuel waits in its action; it does not start off 

 burning, like the lead and many other things that I could 

 show you, but which I have not encumbered the table with; 

 but it waits for action. This waiting is a curious and 

 wonderful thing. Candles those Japanese candles, for in- 

 stance do not start into action at once like the lead or 

 iron (for iron finely divided does the same thing as lead), 

 but there they wait for years, perhaps for ages, without 

 undergoing any alteration. I have here a supply of coal-gas. 

 The jet is giving forth the gas, but you see it does not 

 take fire it comes out into the air, but it waits till it is 

 hot enough before it burns. If I make it hot enough, it 

 takes fire. If I blow it out, the gas that is issuing forth 

 waits till the light is applied to it again. It is curious to see 

 how different substances wait how some will wait till the 

 temperature is raised a little, and others till it is raised a 

 good deal. I have here a little gunpowder and some gun- 

 cotton ; even these things differ in the conditions under which 

 they will burn. The gunpowder is -composed of carbon and 

 other substances, making it highly combustible ; and the gun- 

 cotton is another combustible preparation. They are both 

 waiting, but they will start into activity at different de- 

 grees of heat, or under different conditions. By applying 

 a heated wire to them, we shall see which will start first 

 [touching the gun-cotton with the hot iron]. You see the 

 gun-cotton has gone off, but not even the hottest part of the 

 wire is now hot enough to fire the gunpowder. How 

 beautifully that shows you the difference in the degree in 

 which bodies act in this way ! In the one case the substance 

 will wait any time until the associated bodies are made active 

 by heat; but in the other, as in the process of respiration, it 

 waits no time. In the lungs, as soon as the air enters, it 

 unites with the carbon; even in the lowest temperature 

 which the body can bear short of being frozen, the action 

 begins at once, producing the carbonic acid of respiration; 

 and so all things go on fitly and properly. Thus you see the 

 analogy between respiration and combustion is rendered still 



