THE FIRST AND THE LAST CATASTROPHE. 269 



because a great number of people appear to have been led to the con- 

 clusion that this theory is very similar to the guesses which we find 

 in ancient writers Democritus and Lucretius. It so happens that 

 these ancient writers did hold a view of the constitution of thinirs 

 which in many striking respects agrees with the view which we hold 

 in modern times. This parallelism has been brought recently before 

 the public by Prof. Tyndall in his excellent address at Belfast. And 

 it is perhaps on account of the parallelism, which he pointed out 

 at that place, between the theories held among the ancients and the 

 theory now held among the moderns, that many people who are 

 acquainted with classic literature have thought that a knowledge of 

 the views of Democritus and Lucx'ctius would enable them to under- 

 stand and criticise the modern theory of matter. Tliat, "however, is a 

 mistake. The difference between the two is mainly this : the atomic 

 theory of Democritus was a guess, and no more than a guess. Every 

 body around him was guessing about the origin of things, and they 

 guessed in a great number of ways; but he happened to make a guess 

 which was more near the right thing than any of the others. This 

 view was right in its main hyj^othesis, that all things are made up 

 of elementary parts, and that the different properties of different 

 things depend rather upon difference of arrangement than upon ulti- 

 mate difference in the substance of which they are composed. Al- 

 though this was contained in the atomic theory of Democritus, as 

 expounded by Lucretius, yet it will be found by any one who exam- 

 ines further the consequences which are drawn from it, that it very 

 soon diverges from the truth of things, as we might naturally expect 

 it would. On the contrary, the view of the constitution of matter 

 which is held by scientific men in the present day is not a guess at all. 

 In the first place, I will endeavor to explain what are the main 

 points in this theory. First of all we must take the simplest form 

 of matter, which turns out to be a gas such, for example, as the air 

 in this room. The belief of scientific men in the present day is that 

 this air is not a continuous thing, that it does not fill the w^hole of the 

 space in the room, but is made up of an enormous number of exceed- 

 ingly small pax'ticles. There are two sorts of particles : one sort of 

 particle is oxygen, and another sort of particle nitrogen. All the 

 particles of oxygen are as near as possible alike in these two respects : 

 first in weight, and secondly in certain peculiarities of mechanical 

 structure. These small molecules are not at rest in the room, but are 

 flying about in all directions with a mean velocity of seventeen miles 

 a minute. Tliey do not fly far in one direction ; but any particular 

 molecule, after going over an incredibly short distance the measure 

 of which has been made meets another, not exactly j^bimp, but a 

 little on one side, so that they behave to one another somewhat in 

 the same way as two people do who are dancing Sir Roger de Cover- 

 ley; they join hands, swing round, and then fly away in different 



