i 



8, 9] Historical Note 9 



Historical Note. 



9. The rise of the Kinetic Theory was of a gradual nature, and it is 

 difficult to mention any time at which the theory may be said to have 

 arisen, or any single name to whom honour of its establishment is due. 

 Three stages in its development may be traced. There is first the stage 

 of speculative opinion, unsupported by scientific evidence. Given that 

 a great number of thinkers are speculating as to the structure of matter, 

 it is only in accordance with the laws of probability that some of them 

 should arrive fairly near to the truth. An opinion which turns out 

 ultimately to be near the truth remains, however, of no greater value to 

 the advancement of science, than a more erroneous opinion, until scientific 

 reasons can be given for supposing the former to be more accurate than 

 the latter. When this point is reached the theory may be said to have 

 entered upon the second stage of its development ; the_ true and false 

 opinions are still equally in the field, but the former is supplied with 

 weapons for defeating the latter. In the third stage there is general agree- 

 ment as to the main foundations of the theory and their truth, and labour 

 is devoted no longer to defeating adverse opinion, but to the elaboration of 

 the detail of the theory, and to attempts to extend its boundaries. 



In its earliest stage the growth of the Kinetic Theory is hardly distin- 

 guishable from that of the atomic theory. The view that matter was to be 

 regarded as an aggregation of hard, indivisible and similar parts was upheld 

 by Lucretius, who appears to have taken his opinions from Democritus and 

 Epicurus, who again had been guided by Leucippus. This theory was re- 

 vived by Gassendi in the middle of the seventeenth century*. Apparently 

 Gassendi was the first to suspect that the .motion alone of the atoms was 

 sufficient to account for a number of phenomena, without the introduction 

 of adventitious hypotheses to account separately for these phenomena. 

 Lasswitzf describes Gassendi's work as follows: "Following Democritus and 

 Epicurus, Gassendi in the seventeenth century re-established and elaborated 

 an atomic theory based upon the assumption that all material phenomena 

 can be referred to the indestructible motion of atoms and can therefore 

 be described as " kinetic." Gassendi's atoms are devoid of all qualities 

 except absolute rigidity ; they are similar in substance, but different in size 

 and form, and move in all directions through empty space. On this basis 

 Gassendi explains a number of physical processes, in particular the three 

 states of matter and the transitions from one to another, in a way very 

 little different from that of the modern kinetic theory." It is obvious, 

 then, that with Gassendi the theory is entering upon the second stage of 

 its existence. 



* Syntagma Philosophicum, 1658, Lugduni. 



t "Der Verfall der kinetischen Atomistik im 17 Jahrhnndert," Pogg. Ann. CLHI. p. 373 (1874). 



