WHAT IS ELECTRICITY? 79 



a healthy distrust of our theories, and an abiding faith in that pillar 

 of cloud by day and the pillar of fire by night — the doctrine of the 

 conservation of energy. 



Having thus outlined the present condition of our knowledge, and 

 of our comprehension of the bearing and tendencies of physical science, 

 let us strive, with the most powerful instruments we have, to survey 

 the promised land which is undoubtedly to be the possession of those 

 who come after us. It is one thing to become familiar with all the 

 applications of the mechanical theory of electricity, and another to 

 make an advance in the subject so that we can see the relations of 

 electrical and magnetic attraction to the attraction of gravitation and 

 to what we call chemical attraction. To this possible relationship I 

 wish to call your attention to-day. 



I am forced to believe that the new advances in our knowledge of 

 electrical manifestations are to come from a true conception of the 

 universality of electrical manifestations, and from the advance in the 

 study of molecular physics. Picture to yourselves the position of an 

 investigator in this world. A person on the moon could only conceive 

 of this audience as a molecule made up of many atoms. He could not 

 measure the energy you manifest by moving about — the heat enei'gy 

 — the electrical energy due to the friction of your envelopes. Indeed, 

 he could only suppose your existence, just as we imagine the existence 

 of a molecule of a crystal. Now, the distances we force molecules 

 apart by many of our chemical j^rocesses seem extremely small to us ; 

 but how immense they really are compared with the distances apart 

 of the atoms ! Is it not as if we should take a stone from the moon or 

 from Venus and place it upon the earth in the time of one second ? 

 You can imagine, from the familiar spectacle of a meteor, the heat 

 and the electricity that would result. Yet, in respect to relative dis- 

 tances, do we not do something similar when we break a crystal or 

 pour acid upon a metal, or strike a dynamite-cartridge ? We are in- 

 finitely small ourselves compared with the great universe about us ; 

 yet our task is to comprehend the motions of aggregations of atoms 

 infinitely smaller than that aggregation which we call man. 



When we bi*eak a crystal mechanically, we have a development of 

 electricity. When we heat certain crystals — tourmaline, for example 

 — besides the strain among the molecules of the crystal which is pro- 

 duced by the increased rates of vibration, we have a difference of elec- 

 trical potential. When we let an acid fall from the surface of a metal, 

 the metal takes one state of electrification and the drop of acid the 

 other — in other words, we produce a difference of electrical potential. 

 On the other hand, a difference of electrical potential modifies the ag- 

 gregation of molecules. The experiments of Lippman are well known 

 to you. He has constructed an electrometer, and even an electrical 

 machine, which depend upon the principle that the supei'ficial energy 

 of a surface of mercury covered with acidulated water is modified 



