1 86 THE ADDRESSES, LECTURES, ETC., OF 



electric current by means of a machine (the dynamo-electric 

 machine) which I shall have occasion to describe to you presently, 

 which electric current may in its turn be utilised to produce light 

 such as at present illuminates this hall. By means of the same 

 dynamo-electric apparatus our unit of force could be utilised to 

 cause the separation of chemical compounds, accomplishing, for 

 instance, the deposit of copper of definite amount from its 

 solution. 



It may be said generally that without energy both in its kinetic 

 and potential forms it would be impossible to imagine the very 

 existence of life, of vegetation, and indeed of material creation. 

 It is by molecular or potential energy that the particles of all 

 matter, whether solid, liquid, or gaseous, are maintained in their 

 relative position ; it is by an augmentation of this form of energy 

 that ice is changed into water, and by a further augmentation 

 that water is changed into steam or vapour, giving rise through 

 its withdrawal to rainfall, with all its attendant benefits, to 

 vegetation ; whilst it is by the potential energy residing in coal 

 that we derive warmth, cook our food, and work our factories. 



"Whence, it may be asked, is all this energy derived ? Is it that 

 our earth constitutes herself a mine of potential energy, which we 

 have only to tap and utilise for our purposes ? A little examina- 

 tion into this question will convince us that we have no such 

 store to fall back upon, and that, excepting the coal, there is 

 nothing within our earth to yield us a supply of energy. The 

 water of the ocean is the result of the combustion of hydrogen 

 which may have taken place at some early period of the earth's 

 history, when it must have given rise to an enormous generation 

 of heat ; enough, perhaps, to constitute our planet a luminary 

 body ; but this combustion having been once accomplished, this 

 energy is irrecoverably lost to us, excepting the small remnant which 

 prevents the water from assuming the solid form, and which is 

 unavailable for our purposes. 



If we examine the solid constituents of the earth, such as the 

 siliceous or calcareous rocks, we shall find that they also are the 

 result of former combustion ; in the case of the mountain lime- 

 stone we find that, on heating it, it separates into two substances, 

 calcic oxide, a solid, and carbonic acid, a gaseous body. In 

 examining each of these constituents we find that they also are 



