186 Sir William Thomson [Feb. 2, 



the molecular structure of a building does not necessarily involve the 

 question, Can a brick be divided into parts ? and can those parts be 

 divided into much smaller parts? and so on. It used to be a favourite 

 subject for metaphysical argument amongst the schoolmen whether 

 matter is infinitely divisible, or whether space is infinitely divisible, 

 which some maintained, whilst others maintained only that matter is 

 not infinitely divisible, and demonstrated that there is nothing in- 

 conceivable in the infinite subdivision of space. Why, even time was 

 divided into moments (time-atoms !), and the idea of continuity of 

 time was involved in a halo of argument, and metaphysical — I will 

 not say absurdity — but metaphysical word-fencing, which was no 

 doubt very amusing for want of a more instructive subject of study. 

 There is in sober earnest this very important thing to be attended to, 

 however, that in chronometry, as in geometry, we have absolute con- 

 tinuity, and it is simply an inconceivable absurdity to suppose a limit 

 to smallness whether of time or of space. But on the other hand, 

 whether we can divide a piece of glass into pieces smaller than the 

 l-100,000th of a centimetre in diameter, and so on without breaking 

 it up, and making it cease to have the properties of glass, just as a 

 brick has not the property of a brick wall, is a very practical question, 

 and a question which we are quite disposed to enter upon. 



I wish in the beginning to beg you not to run away from the 

 subject by thinking of the exceeding smallness of atoms. Atoms are 

 not so exceedingly small after all. The four lines of argument I have 

 referred to make it perfectly certain that the molecules which consti- 

 tute the air we breathe are not very much smaller, if smaller at all, 

 than 1-10, 000, 000th of a centimetre in diameter. I was told by a 

 friend just five minutes ago that if I give you results in centimetres 

 you will not understand me. I do not admit this calumny on the 



Fig. 1. 



One centimetre. One millimetre. 



Eoyal Institution of Great Britain ; no doubt many of you as English- 

 men are more familiar with the unhappy British inch ; but you all 

 surely understand the centimetre, at all events it was taught till a few 

 years ago in the primary national schools. Look at that diagram 

 (Fig. 1), as I want you all to understand an inch, a centimetre, a milli- 

 metre, the 1-lOth of a millimetre, and the 1-lOOth of a millimetre, 

 the 1-lOOOth of a millimetre, and the l-l,000,000th of a millimetre. 

 The diagram on the wall represents the metre ; below that the yard ; 

 next the decimetre, and a circle of a decimetre diameter, the centi- 

 metre and a circle of a centimetre, and the millimetre, which is 1-lOth 

 of a centimetre, or in round numbers l-40th of an inch. We will 



