440 



SCIENCE. 



[N. S. Vol. XIV. No. 351. 



ical bodies in the particular case in which the 

 sizes are distributed according to the law 

 of errors, which would involve far greater 

 inequalities than can occur among atoms. 

 Without discussing the precise details of 

 his problem it is sufficient to say that in 

 the case considered by him the length of 

 the mean free path is /y of what it would 

 be if the particles were equal. Hence were 

 the inequalities of atoms as great as in this 

 extreme case, the reduction of the mean 

 free path in hydrogen could only be from 

 185 to 119 iJ-iJ. ; but they must be far less, 

 and therefore the error, if any, due to this 

 cause could not approach this amount. It 

 is probably inappreciable. 



Such examples might be multiplied but 

 the one I have selected is perhaps sufficient 

 to illustrate my point, viz., that consider- 

 able and fairly accurate knowledge can be 

 obtained as to molecular quantities by the 

 aid of theories the details of which are 

 provisional, and are admittedly capable of 

 improvement. 



IS THE MODEL UNIQUE ? 



But the argument that a correct result 

 may sometimes be obtained by reasoning 

 on imperfect hypotheses raises the question 

 as to whether another danger may not be 

 imminent. To be satisfactory our model 

 of nature must be unique, and it must be 

 impossible to imagine any other which 

 agrees equally well with the facts of experi- 

 ment. If a large number of hypotheses 

 could be framed with equal claims to valid- 

 ity, that fact would alone raise grave 

 doubts as to whether it were possible to 

 distinguish between the true and the false. 

 Thus Professor Poincare has shown that 

 an infinite number of dynamical explana- 

 tions can be found for any phenomenon 

 which satisfies certain conditions. But 

 though this consideration warns us against 

 the too ready acceptance of explanations 

 of isolated phenomena, it has no weight 



against a theory which embraces so vast a 

 number of facts as those included by the 

 atomic theory. It does not follow that, be- 

 cause a number of solutions are all formally 

 dynamical, they are therefore all equally ad- 

 missible. The pressure of a gas may be 

 explained as the result of a shower of blows 

 delivered by molecules, or by a repulsion 

 between the various parts of a continuous 

 medium. Both solutions are expressed in 

 dynamical language ; but one is, and the 

 other is not, compatible with the observed 

 phenomena of expansion. The atomic the- 

 ory must hold the field until another can 

 be found which is not inferior as an expla- 

 nation of the fundamental difficulties as to 

 the constitution of matter, and is, at the 

 same time, not less comprehensive. 



On the whole, then, the question as to 

 whether we are attempting to solve a 

 problem which has an infinite number of 

 solutions may be put aside until one solu- 

 tion has been found which is satisfactory in 

 all its details. We are in a sufficient diffi- 

 culty about that to make the rivalry of a 

 second of the same type very improbable. 



THE PHENOMENA OF LIFE. 



But it may be asked — nay, it has been 

 asked — may not the type of our theories 

 be radically changed ? If this question does 

 not merely imply a certain distrust in our 

 own powers of reasoning, it should be sup- 

 ported by some indication of the kind of 

 change which is conceivable. 



Perhaps the chief objection which can be 

 brought against physical theories is that 

 they deal only with the inanimate side of 

 nature, and largely ignore the phenomena 

 of life. It is therefore in this direction, if 

 in any, that a change of type may be ex- 

 pected. I do not propose to enter at length 

 upon so difficult a question, but, however 

 we may explain or explain away the char- 

 acteristics of life, the argument for the 

 truth of the atomic theory would only be 



