12 REPORT— 1887. 



ill the original material, may have accumulated to a different extent in 

 the various fractious, their presence being indicated by the only test by 

 which they can now be detected. Which of these three explanations is 

 the true one must be left to future experiment to decide. 



We mnst now pass from the statics to the dynamics of chemistry ; that 

 is, from the consideration of the atoms at rest to that of the atoms in 

 motion. Here again we are indebted to John Dalton for the first step 

 in this direction, for he showed that the particles of a gas are constantly 

 flying about in all directions; that is, that gases diffuse into one another, 

 as an escape of coal gas from a burner, for example, soon makes itself 

 perceptible throughout the room. Dalton, whose mind was constantly 

 engaged in studying the molecular condition of gases, first showed that 

 a light gas cannot rest upon a heavier gas as oil upon water, but that an 

 in terpen etration of each gas by the other takes place. It is, however, to 

 Graham's experiments, made rather more than half a century ago, that 

 we are indebted for the discovery of the law regulating these molecular 

 motions of gases, proving that their relative rates of diffusion are inversely 

 proportional to the square roots of their densities, so that oxygen being 16 

 times heavier than hydrogen, their relative rates of diffusion are 1 and 4. 



But whilst Dalton and Graham indicated that the atoms are in a con- 

 tinual state of motion, it is to Joule that we owe the first accurate deter- 

 mination of the rate of that motion. At the Swansea Meeting in 1848, 

 Joule read a paper before Section A on the Mechanical Equivalent of 

 Heat and on the Constitution of Elastic Fluids. In this paper Joule 

 remarks that whether we conceive the particles to be revolving round 

 one another according to the hypothesis of Davy, or flying about in 

 every direction according to Herapath's view, the pressure of the gas will 

 be in proportion to the vis viva of its particles. * Thus it may be shown 

 that the particles of hydrogen at the barometrical pressure of 30 inches 

 at a temperature of 60° must move with a velocity of 6225 54 feet per 

 second in order to produce a pressure of 14*714 lbs. on the square inch ; ' 

 or, to put it in other words, a molecular cannonade or hailstorm of parti- 

 cles, at the above rate — a rate, we must remember, far exceeding that 

 of a cannon ball — is maintained against the bounding surface. 



We can, however, go a step further and calculate with Clerk Maxwell 

 the number of times in which this hydrogen molecule, moving at the rate 

 of 70 miles per minute, strikes against others of the vibrating swarm, 

 and we learn that in one second of time it must knock against others no 

 less than 18 thousand million times. 



And here we may pause and dwell for a moment on the reflection that 

 in nature there is no such thing as great or small, and that the structure of 

 the smallest particle, invisible even to our most searching vision, may be 

 as complicated as that of any one of the heavenly bodies which circle round 

 our sun. 



But how does this wonderful atomic motion affect our chemistry ? 

 Can chemical science or chemical phenomena throw light upon this 



