456 Prof. Williamson on the Dynamics 



equal in density to atmospheric air. At the other side of the 

 porous disc which closes the mouth of the bell jar, will be a 

 similar jar marked by a black band, containing common air; and 

 if we get red fumes in the jar with a black rim, that will be a 

 proof that nitric oxide has come out and mixed with the air in 

 that black jar. If, on the other hand, we get red fumes in the 

 jar with the white rim, that will prove to us that air has got into 

 that jar and mixed with the nitric oxide contained in it. You 

 see by the result that each gas is moving through the porous 

 septum. I must now go a step further, and ask you to follow 

 me into some chemical considerations — in fact the consideration 

 of that wonderful process which it has been the chief business, 

 and, I may add, the glory of chemistry to explain, I mean the 

 process of combustion. We chemists think of combustion mainly 

 as a process of combination of different materials with each 

 other, forming compounds differing from those materials in their 

 properties. But much better for our present purpose is the 

 popular view of combustion, which consists in looking upon it as 

 a plan for getting heat. You really do not burn coals in order 

 to get carbonic acid and water by combining the carbon and 

 hydrogen with oxygen from the air ; but you burn them in order 

 to heat your houses, or your factories, or steam-boilers, &c. 

 Combustion is the process by which some of the inherent mo- 

 tion of atoms is taken from them on combining, and transferred 

 in the form of heat to surrounding objects. If you burn a hun- 

 dredweight of coal, and collect all the heat which is given off by 

 its complete combustion, leaving the products as cold as the ori- 

 ginal materials before use, you have enough heat to boil 

 about 80 hundredweight of water, that is, to heat 80 hundred- 

 weight from 32° F. to 212° F. Now in such an experiment we 

 may say that all this heat is taken out of the coals and oxygen by 

 merely combining them, and accordingly the products contain 

 just so much less heat, or the motion which constitutes heat, than 

 the original materials before combination. 



If we burn iron, as it very often is burnt in forges, we have 

 heat given off, and the oxide of iron contains just that much 

 heat less than the iron and oxygen before combination. Pre- 

 cisely the same holds good when I break this little bulb, and 

 allow the iron which is contained in it to burn in the chlorine 

 contained in this tall cylinder. You see evidences of the heat 

 which is given off by the direct formation of the chloride of iron 

 from its elements ; and for every pound of chlorine used to form 

 this compound, there are 1492 degrees of heat evolved, assuming 

 that the experiment is performed in the presence of water. 



If chlorine is used to burn copper in the same manner, form- 

 ing protochloride of copper, there are 962 degrees of heat evolved 



