Now, when a person is setting off to discuss the relation between electricity and light, it is very natural and very proper to pull him up short with the two questions: What do you mean by electricity? and What do you mean by light? These two questions I intend to try briefly to answer. And here let me observe that in answering these fundamental questions, I do not necessarily assume a fundamental ignorance on your part of these two agents, but rather the contrary; and must beg you to remember that if I repeat well-known and simple experiments before you, it is for the purpose of directing attention to their real meaning and significance, not to their obvious and superficial characteristics; in the same way that I might repeat the exceedingly familiar experiment of dropping a stone to the earth if we were going to define what we meant by gravitation. Now, then, we will ask first, What is electricity? and the simple answer must be, We don't know. Well, but this need not necessarily be depressing. If the same question were asked about matter, or about energy, we should have likewise to reply, No one knows. But then the term Matter is a very general one, and so is the term Energy. They are heads, in fact, under which we classify more special phenomena. Thus, if we were asked, What is sulphur? or what is selenium? we should at least be able to reply, A form of matter; and then proceed to describe its properties, _i. e._, how it affected our bodies and other bodies. Again, to the question, What is heat? we can reply, A form of energy; and proceed to describe the peculiarities which distinguish it from other forms of energy. But to the question. What is electricity? we have no answer pat like this. We can not assert that it is a form of matter, neither can we deny it; on the other hand, we certainly can not assert that it is a form of energy, and I should be disposed to deny it. It may be that electricity is an entity _per se_, just as matter is an entity _per se_. Nevertheless, I can tell you what I mean by electricity by appealing to its known behavior. Here is a battery, that is, an electricity pump; it will drive electricity along. Prof. Ayrtou is going, I am afraid, to tell you, on the 20th of January next, that it _produces_ electricity; but if he does, I hope you will remember that that is exactly what neither it nor anything else can do. It is as impossible to generate electricity in the sense I am trying to give the word, as it is to produce matter. Of course I need hardly say that Prof. Ayrton knows this perfectly well; it is merely a question of words, _i. e._, of what you understand by the word electricity.