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LV. On the Law of Gaseous Pressure. By the Hon. J. W. 



Strutt, late Fellow of Trinity College } Cambridge. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



ABSENCE from England has prevented my seeing Mr. 

 Moon's last paper (Phil. Mag. Feb. 1873) until lately. 

 I question whether there would be any use in continuing the 

 controversy, as we seem to have hardly any common ground for 

 argument ; but Mr. Moon asks me one or two definite questions 

 which I ought not to leave unanswered. 



I am asked whether I still <c consider that Boyle's law has been 

 experimentally proved in the case of motion/' If forced to give 

 a categorical answer, I must say yes ; for the simple reason that 

 all the experiments on gases ever made relate to the case of mo- 

 tion. Since my first note on this subject, Mr. Moon has ex- 

 plained, or at least admitted my assertion, that the absolute 

 velocity of a gas is not material; so that in, fact by motion he 

 means relative motion of the various parts of a gas. I am free 

 to admit that the experiments by which Boyle's law is established 

 do not extend to the case of relative motion. 



Mr. Moon finds a reductio ad absurdum of the received law of 

 pressure in an argument relating to the behaviour of air confined 

 under a piston when a weight is placed on the latter, and con- 

 tends that the fallacy consists in the false assumption of the 

 received law. I took some pains to point out that it is not 

 merely the truth or falsehood of Boyle's law (as extended) which 

 is at issue, but that Mr. Moon's argument, if valid at all, goes 

 the length of proving that it is impossible, without self-contra- 

 diction, even to conceive a medium w T hose pressure shall (under 

 all circumstances) vary as the density. My position is that the 

 fallacy or absurdity lies not in the premises, but in the argu- 

 ment, which I applied to prove that a body would not fall when 

 its support is removed. Mr. Moon does not admit the paral- 

 lelism of the two paradoxes, and asks me to point out exactly 

 where the fallacy lies. To enter into a complete explanation 

 would be to write a dissertation on the principles of the differen- 

 tial calculus, for which this is certainly not the place ; but I 

 may say that (in my opinion) the error lies in the omission 

 of the word finite. There can be no finite change of pressure 

 without a finite displacement, nor can there be a finite displace- 

 ment without a finite change of pressure. This much is admitted ; 

 but it does not follow that there is never a finite displacement 

 or change of pressure. In fact the quantities become finite 



