2 Mr. J. Croll on What determines Molecular Motion ?— 



our conceptions by finding one of a higher order — and so on, ever 

 trying to reach the highest unity^ the most general conception 

 possible. 



The point, however^ which more immediately claims our spe- 

 cial attention at present is this ; — As the physical sciences pro- 

 ceed in their generalization, they advance more and more towards 

 Molecular Physics. We may illustrate our meaning by examples 

 without number from any of the sciences. In Electricity, for 

 example, all the ordinary questions — such as how a Ley den jar 

 becomes charged, or how the electricities are supposed to be 

 decomposed on a conductor, or by what means one body charged 

 positively will act upon another body charged negatively — were 

 formerly considered to be answered quite satisfactorily without 

 making any reference whatever to the molecular condition of the 

 bodies under the electric influence. But when we come to in- 

 quire more deeply into what is meant by induction— what that 

 peculiar condition is which constitutes the charged jar, and the 

 nature of that hidden change which takes place on the conductor 

 while what we call its electricities are being decomposed, — we 

 begin to find that we are entering upon deep and difficult ques- 

 tions regarding the hidden operations taking place among the 

 molecules or the atoms of the electrified body. Electricity was 

 formerly supposed to be a fluid substance altogether distinct 

 from the body in which it manifested itself; and, of course, in- 

 quiry was directed towards this hypothetical substance, and not 

 to the molecular condition of the body. But the grounds for 

 believing in the existence of this hypothetical fluid are fast dis- 

 appearing ; and electricity is now generally believed to be a con- 

 dition or motion of the molecules or the atoms of the electrified 

 body itself. Similarly, heat at one time was considered to be a 

 substance to which the name Caloric was applied. It is now, 

 however, proved to be not a substance, but a particular mode of 

 motion of the atoms of the heated body. The same is proved to 

 be the case in regard to Light ; and Magnetism, which was for- 

 merly explained by means of hypothetical fluids, is now believed 

 to consist also in a particular molecular condition of the mag- 

 nietic body ; whilst Chemistry is fast becoming a question re- 

 garding the dynamical actions of the atoms of the combining 

 substances. 



In Physical Astronomy questions regarding the constitution 

 of the sun, the fixed stars, and nebulae are being determined by 

 Molecular Physics; and even the dia-ection and velocity of their 

 motions are now resolved by the same method. Physical in- 

 quiry in every direction is converging towards Molecular Phy- 

 sics, is resolving itself into questions regarding the dynamical 

 action of the ultimate particles of matter. 



