of the Newtonian Lata of Molecular Action. 439 



when the density and arrangement of the aether are changed 

 by the introduction of particles of matter which exclude some 

 particles of aether from certain portions of space, and thereby 

 affect the equilibrium positions of the remaining particles of 

 aether. The Professor next endeavours to guess the reason 

 why I did not draw from my equations any inferences respect- 

 ing the direct action of matter. The answer is simple ; a most 

 important step required that the vibrating medium should 

 consist of homogeneous particles. The step I allude to is that 

 where (S. 3. vol. xx. May, p. 372) I have " assumed the law of 

 displacement at the time t to be £ r = a sin (r h + T)," which 

 assumption is not true when the particles of matter vibrate, be- 

 cause then a could not be invariable through the medium. I 

 think no blame attaches to me for causing this perplexity to the 

 Professor, as I have expressly added, " it will be understood 

 that what follows applies only to media in which this law of 

 disturbance can be transmitted," which I understand to be a 

 formal renunciation of all connexion with the direct action of 

 matter.] The Professor, therefore, in referringme to Mr. O' Brien 

 (October, p. 269) to be set right in my notions, seems to have 

 fallen into the mistake of supposing that what Mr. O'Brien 

 has written on the direct effect of matter (March, note at 

 p. 208) can refute what I have written on the indirect effect 

 of matter. 



I do not think I fully understand on what ground the Pro- 

 fessor affirms (October, p. 264) that I have not taken account 

 of " the *want of action of particles of aether in the portion of 

 space occupied by the material particles" (October, p. 264); 

 for, as I have taken into account all those particles of aether 

 which do exist, and none else that I am aware of, I sup- 

 pose I must have omitted those which do not exist. Perhaps 

 the Professor will point out what step of my investigation 

 implies the existence of the absent particles. 



I am next accused (p. 264) of not saying a word about 

 " the pressure of the particles of matter on the adjacent parti- 

 cles of aether tending to stop their motion." In reply it seems 

 sufficient to state, that the particles of matter are supposed to 

 be so few in number in comparison with the particles of aether 

 in a refracting medium, that though a wave may in some de- 

 gree be broken up in its passage through the medium by 

 material impediments, the sensible properties of its general 

 front will remain almost, if not entirely unaffected ; wherefore 

 in an argument based on the broad features of refraction, any 

 allusion to this consideration were a useless refinement, a 

 needless entering upon difficulties, and an unnecessary inter- 



