L\, I INTRODUCTORY LBCTUBK ON EXPERIMENTAL PHYSICS. 



I hope to be able to lay before you in the course of the term some of 

 tho evidence for the existence of molecules, considered as individual bodies 

 having definite properties. The molecule, as it is presented to the scientific 

 imagination, is a very different body from any of those with which experience 

 lias hitherto made us acquainted. 



In the first place its mass, and the other constants which define its 

 properties, are absolutely invariable ; the individual molecule can neither grow 

 nor decay, but remains unchanged amid all the changes of the bodies of which 

 it may form a constituent. 



In the second place it is not the only molecule of its kind, for there are 

 innumerable other molecules, whose constants ore not approximately, but abso- 

 lutely identical with those of the first molecule, and this whether they ore 

 found on the earth, in the sun, or in the fixed stars. 



By what process of evolution the philosophers of the future will attempt 

 to account for this identity in the properties of such a multitude of bodies, 

 each of them unchangeable in magnitude, and some of them separated from 

 others by distances which Astronomy attempts in vain to measure, I cannot 

 conjecture. My mind is limited in its power of speculation, and I am forced 

 to believe that these molecules must have been made as they are from the 

 beginning of their existence. 



I also conclude that since none of the processes of nature, during their 

 varied action on different individual molecules, have produced, in the course of 

 ages, the slightest difference between the properties of one molecule and those 

 of another, the history of whose combinations has been different, we cannot 

 ascribe either their existence or the identity of their properties to the operation 

 of any of those causes which we coll natural. 



Is it true then that our scientific speculations have really penetrated 

 beneath the visible appearance of things, which seem to be subject to gene- 

 ration and corruption, and reached the entrance of that world of order and per- 

 fection, which continues this day as it was created, perfect in number and 

 measure and weight ? 



We may be mistaken. No one has as yet seen or handled an individual 

 molecule, and our molecular hypothesis may, in its turn, be supplanted by some 

 new theory of the constitution of matter; but the idea of the existence of 

 unnumbered individual things, all alike and all unchangeable, is one which cannot 

 enter the human mind and remain without fruit. 



