468 Dr. G. J. Stoney on Texture in Media, and on the 



divided for the purposes of the integration may be made as 

 small as we please without ceasing to be subjected to the law 

 in heavy liquids of pressure proportional to the depth below 

 the upper surface of the water, plus that due to the superin- 

 cumbent atmosphere. 



Both these assumptions continue approximately true when 

 the elements into which we suppose the surface divided are 

 diminished till they are as small as, or even a good deal 

 smaller than, the smallest speck that can be distinguished with 

 the most powerful microscope ; but they utterly break down 

 if we suppose the subdivision carried so much further as to 

 reach or even approach the scale of molecular magnitudes. 

 If, for instance, the elements into which we suppose the sur- 

 face divided were reduced to a square tenth-metret * in size, 

 a patch of surface which is the millionth part of the utmost a 

 microscopist can see, we should have got well within the 

 range f of molecular differences. The boundary between the 

 water and sluice would cease to be a surface : it would be the 

 continually shifting boundary between molecules on both 

 sides in energetic motion, acting individually on each other 

 in their own special ways ; which happen to be such that when 

 immense numbers of these individual operations are lumped 

 together they produce approximately, as the outcome of all 

 that is going on, that law of pressure proportional to depth 

 with which we are familiar. 



Thus, what we regard as a physical property of the medium 

 — in this case the law of pressure in a heavy liquid — is in 

 reality a statement of what is the drift of a vast number of 

 individual events, grouped together by a kind of statistical 

 process. This we may briefly describe by saying that the 

 dynamical properties of the medium are due to its texture, 



* The decimetre is the first of the metrets (i. e. decimal subdivisions of 

 the metre), the centimetre is the second, the millimetre is the third. The 

 tenth-metret is the tenth of this series. It is a metre divided by 10 10 . 

 The waves of visible light have lengths varying from S900 to 7600 of 

 these tenth-metrets. 



t According to Professor Loschmidt, who first published an estimate 

 of the interval within which the centres of two molecules must approach 

 to act sensibly on one another, this interval is about a ninth-metret 

 (Proceedings of the Mathematical Section of the Academy of Vienna, Oct. 

 1865, p. 404). The mean of such intervals may, perhaps with more pro- 

 bability, be taken as lying nearer to the tenth-metret. It is very impro- 

 bable that it is as small as the eleventh-metret. In the present paper 

 I assume it to be about the tenth-metret. If, however, it lies nearer the 

 ninth-metret, though we shall have to change almost all the numbers of 

 the computation in the text, we shall arrive at a conclusion not materially 

 differing from that on p. 472 below. 



