394 Scientific Proceedings, Royal Dublin Society. 



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 

 ve may briefly describe by saying that the dynamical properties 

 of the medium are due to its texture, meaning by the texture of a 

 nedium whatever is going on in it at close quarters. 



It is the same with the other recognised physical properties of 

 media, such as what are called gaseous laws — the laws connect- 

 ing the density, temperature, and pressure of gases, the laws of 

 their diffusion, the law of viscosity, and so on. Similarly with the 

 properties of solid bodies : all are the outcome of vast numbers of 

 very diverse individual events that occur between or within the 

 molecules of the bodies, or between them and the luminiferous 

 ether. 



In order to penetrate to this world of individual actions, we 

 must not only descend to magnitudes that are comparable to the 

 intervals at which the centres of molecules are spaced from one 

 another, but we must also consider periods of time that are not too 

 vast in reference to the motions that go on among them. A second 

 of time is " out of all whooping" too long;l//) but a period which is 

 definite and of suitable brevity is that for which I have elsewhere 

 proposed the symbol t — viz., the time that light takes to advance 

 one millimetre in vacuo. The velocity of light being 30 quadrants 

 per second, and the quadrant (the length of a meridian from the 

 Earth's equator to the pole) being 10 10 millimetres, we find that 



1 one second 



T = 



10 1 



(d) See the Philosophical Magazine for August, 1868, p. 140, footnote. Readers 

 of the Paper here referred to are requested to change the square of 16 in the second 

 paragraph into the square root of 16. In that Paper, p. 141, I estimated the number 

 of molecules in a cubic millimetre of a gas, at atmospheric temperatures and pressures, 

 as about a uno-eighteen (10 18 ), -without being aware that a similar estimate had been 

 obtained for solids and liquids by Professor Loschmidt in 1865 (Proceedings of the 

 Mathematical Section of the Academy of Vienna for October, 1865, p. 405). In March, 

 1870, Sir "William Thomson, doubtless also without knowing of what had been done 

 before, published a paper in Nature on the " size of atoms," and arrives at substantially 

 the same estimates as Professor Loschmidt and myself. 



The earliest determination of a molecular magnitude, so far as I am aware, was that 

 made by Professor Clerk Maxwell of the mean length of the "free paths" of the 



