THE ATOMIC VIEW OF NATURE. 



437 



37. 

 The atomic 



physical 

 theory about 

 1800. 



convenient symbolism, had, about the year 1860, been 

 accepted by physicists, and used not merely as a con- ^gptg^^g'^ 

 venient symbolism, but as a physical reality. 



Joule had actually calculated the velocity of a particle 

 of hydrogen gas. The atomic view of nature was now 

 taken in real earnest. To establish it still further, there 

 were required definite numerical data ^ as to the size of 

 the smallest particles (henceforth sometimes called atoms, 

 sometimes more correctly molecules) and their number, 

 and also clearer views as to the composition of the mole- 

 cules out of their elements, the chemical atoms. 



The interest which attaches to this latest development 

 of the atomic theory is very great : it has brought about 

 a union of the researches of chemists and physicists, and 

 has made chemistry a province of natural philosophy.^ No 

 one has done more that the late Professor Clerk Max- 



^ Numerical data regarding the 

 size and number of smallest physi- 

 cal particles contained in a given 

 volume of matter have been sup- 

 plied by various methods or various 

 "lines of reasoning." The best 

 summary will he found in Lord 

 Kelvin's lecture, " On the Size of 

 Atoms" (1883 : reprinted in 'Pop- 

 ular Lectures and Addresses,' vol. 

 i. p. 147 sqq.) The four lines of 

 reasoning are founded on the un- 

 dulatory theory of light, on the 

 phenomena of contact electricitj', 

 on capillary attraction, and on the 

 kinetic theory of gases. They 

 " agree in showing that the mole- 

 cules of ordinary matter must be 

 something like the one ten - mil- 

 lionth, or from the one ten -millionth 

 to the one hundred-millionth of a 

 centimetre in diameter." 



^ " We can distinguish two kinds 

 of motion, atomic motion and mole- 

 cular motion. ... To this dis- 



tinction corresponds the division 

 of natural philosophy into physics 

 and chemistry^, not rigidly, yet in 

 so far as chemistry is mainly oc- 

 cupied with the equilibrium of the 

 atoms, physics with the mechanics 

 of the molecules. Chemical equil- 

 ibrium, unchanged condition of the 

 molecules, exists if the affinity 

 which holds together the atoms 

 equilibrates the forces which tend 

 to loosen the composition of the 

 molecule : these forces consist in 

 the motion of the atoms. ... As 

 accordingly in a chemically stable 

 compound the atomic motions re- 

 main in lasting dynamical equil- 

 ibrium with the chemical forces, . . . 

 there remains for the examination 

 of the purely physical phenomena 

 in the first instance only the mole- 

 cular movements" (0. E. Meyer, 

 ' Die kinetische Theorie der Gase,' 

 vol. i. p. 6). 



