equation. Now, that point is given definitely as a point of time, not with great accuracy, but still as near as we can expect to get it, with such means of measuring as we have, and Sir William Thompson has calculated that the earth must have solidified at some time a hundred millions or two hundred millions of years ago; and there we arrive by a present state of things at the beginning of the process of cooling the earth which is going on now. Before that it was cooling as a liquid, and in passing from the liquid to the solid state there was a catastrophe which introduced a new rate of cooling, so that by means of that law we do come to a time when the earth began to assume the present state of things—not that of the existence of the universe at all; we do not give the time of the commencement of the universe, but simply the structure of the earth. If we went farther back, we might make a further calculation and find how long the earth had been in a liquid state. We should come to another catastrophe, and say at that time, not that the universe began to exist, but that the present earth passed from the gaseous to the liquid state. And if we went farther back still we should probably find the earth falling together out of a great ring of matter surrounding the sun, and distributed over its orbit. The same thing is true of every body of matter: if we trace its history back, we come to a certain time at which the catastrophe took place, and if we were to trace back the history of all the bodies of the universe in that way we should continually see them separating up, and falling together, as they have done. What they have actually done is to fall together and get solid. If we should reverse the process we should see them separating and getting cool, and, as a limit to that, we should find that all these bodies would be resolved into molecules, and all these would be flying away from each other. There would be no limit to that process, and we could trace it as far back as ever we liked to trace it. So that on the assumption, a very large assumption, that the present constitution of the laws of geometry and mechanics has held good during the whole of past times, we should be led to the conclusion that at an inconceivably long time ago the universe did consist of ultimate molecules, all separate from one another, and approaching one another, because we have to reverse our former process. Instead of their being at a great distance from one another, and all traveling toward some place where they would meet, the reverse would be the case. Then you would have the process of chlorine going on in these bodies, exactly as we find it going on now, but you will observe that we do not come to such a catastrophe as implies that we have to stop these laws of Nature. We come to something of which we cannot make any further calculation; we find that, however far we like to go back, we approximate to that actual state of things, but never actually get to it. Here we have a doctrine about the beginning of things. First, we have a probability, about as correct as science can make it, of the
