Stoney — On Texture in Media, etc. 393 



pressure, we integrate over the surface between the water and the 

 sluice, and in doing so assume — 



1° that the boundary is a surface ; and 



2° that the elements into which we conceive this surface 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 super- 

 incumbent 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 spec that can be distinguished with the most powerful 

 microscope ; but they utterly break down if we suppose the subdi- 

 vision carried so much farther as to reach or even approach the 

 scale of molecular magnitudes. If, for instance, the elements into 

 which we suppose the surface divided were reduced to a square 

 tenth-metret (6) 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 (c) of molecular differences. The boundary between the 

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

 nually shifting boundary between molecules on both sides in ener- 

 getic 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 — 



(b) 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 3900 to 7600 of these tenth-metrets. 



(c) 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, October, 1865, p. 404). The mean of such intervals may, 

 perhaps with more probability, be taken as lying nearer to the tenth-metret. It is 

 very improbable 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, 

 we shall have to change 80,000 into 800 on p. 398. 



2H2 



