9O THE ADDRESSES, LECTURES, ETC., OF 



how far we are still removed in our practice from this limit of 

 perfection. 



The following illustrations will show the convertibility of the 

 different forms of energy. If I let the weight of a hammer 

 descend in rapid succession upon a piece of iron it becomes hot, 

 and on beating a nail thus vigorously and skilfully for a minute it 

 will be red hot. In this case the mechanical force developed in 

 the arm (by the expenditure of muscular fibre) is converted into 

 heat. Again, in rapidly compressing the air in a fire syringe, 

 ignition of a piece of tinder is obtained. Again, in passing an 

 electrical current through the platinum wire it is directly converted 

 into heat, which is manifested by ignition of the wire, whereas the 

 thermopile gives an illustration of the conversion of heat into 

 electricity ; to which illustrations many others might be added. 

 The heat of combustion being the result of the chemical com- 

 bination of two substances, does it not follow that oxygen is a 

 combustible as well as a carbonaceous substance which goes by the 

 name of fuel ? This is, unquestionably, the case, and if our 

 atmosphere was composed of a carbonaceous gas we should have 

 to conduct our oxygen through tubes and send it out through 

 burners to supply us with light and heat, as will be seen by the 

 experiment in which I burn a jet of atmospheric air in a trans- 

 parent globe filled with common lighting gas ; but we could not 

 exist under such inverted conditions, and may safely strike out 

 oxygen and analogous substances such as chlorine from the list of 

 fuels. 



We now approach the second part of our inquiry 



WHENCE is FUEL DERIVED ? 



The rays of the sun represent energy in the form of heat and 

 light, which is communicated to our earth through the transparent 

 medium which must necessarily fill the space between us and our 

 great luminary. If these rays fall upon the growing plant, their 

 effect disappears from direct recognition by our senses, inasmuch 

 as the leaf does not become heated as it would if it were made of 

 iron or dead wood^but we find a chemical result accomplished, 

 viz., carbonic acid gas, which has been absorbed by the leaf of the 

 tree from the atmosphere, is there " dissociated, 11 or separated 



