476 



NA TURE 



[September 12, 1901 



It may, however, be argued that these concordant values are 

 all obtained by means of the same theory, and that a common 

 error may affect them all. In particular, some critics have of 

 late been inclined to discredit the atomic theory by pointing out 

 that the strong statements which have sometimes been made as 

 to the equality, among themselves, of atoms or molecules of the 

 same kind may not be justified, as the equality may be that of 

 averages only, and be consistent with a considerable variation in 

 the sizes of individuals. 



Allowing this argument more weight than it perhaps deserves, 

 it is easy to show that it cannot affect seriously our knowledge 

 of the length of the mean free path. 



Prof. George Darwin (Phil. Trans., l8o) has handled the 

 problem of a mixture of unequal spherical bodies in the par- 

 ticular 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 7/1 1 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 iiQiuu; 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 

 considerable 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 improve- 

 ment. 



Is the Model Uni,jiie f 

 But the argument that a correct result may sometimes be ob- 

 tained by reasoning on imperfect hypotheses raises the question 

 as to whether another danger m.iy 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 experiment. If a large number of hypotheses could 

 be framed with equal claims to validity, that fact would alone 

 raise grave doubts as to whether it were possible to distinguish 

 between the true and the false. Thus Prof. Poincare has shown 

 that an infinite number of dynamical explanations can be found 

 for any phenomenon which satisfies certain conditions. But 

 though this consideration warns us against the too ready accept- 

 ance 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, 

 because a number of solutions are all formally dynamical, they 

 are therefore all equally admissible. 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 con- 

 tinuous 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 theory must 

 hold the field until another can be found which is not inferior as 

 an explanation of the fundamental difficulties as to the constitu- 

 tion of matter, and is, at the same time, not less compre- 

 hensive. 



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 solution has been found 

 which is satisfactory in all its details. We are in a sufficient 

 difficulty about that to make the rivalry of a second of the same 

 type very improbable. 



The Phciioiiieita 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 reason- 

 ing, it should be supported 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 there- 

 fore in this direction, if in any, that a change of type may be 

 expected. I do not propose to enter at length upon so difficult 

 a question, but, however we may explain or explain away the 

 characteristics of life, the argument for the truth of the atomic 

 theory would only be affected if it could be shown that living 

 matter does not possess the thermal and mechanical properties, 



NO. 1663, VOL. 64] 



to explain which the atomic theory has been framed. This is 

 so notoriously not the case that there is the gravest doubt 

 whether life can in any way interfere with the action within the 

 organism of the laws of matter in bulk belonging to the domain 

 of mechanics, physics, and chemistry. 



Probably the most cautious opinion that could now be ex- 

 pressed on this question is that, in spite of some outstanding 

 difiiculties which have recently given rise to what is called Neo- 

 vitalism, there is no conclusive evidence that living matter can 

 suspend or modify any of the natural laws which would affect it 

 if it were to cease to live. It is possible that though subject to 

 these laws the organism while living may be able to employ, or 

 even to direct, their action within itself for its own benefit, just 

 as it unquestionably does make use of the processes of external 

 nature for its own purposes ; but if this be so, the seat of the 

 controlling influence is so withdrawn from view that, on the one 

 hand, its very existence may be denied, while, on the other 

 hand. Prof. Haeckel, following Vogt, has recently asserted that 

 " matter and ether are not dead, and only moved by extrinsic 

 force ; but they are endowed with sensation and will ; they ex- 

 perience an inclination for condensation, a dislike for strain ; 

 they strive after the one and struggle against the other " 

 (" Riddle of the Universe," English translation, 1900, p. 380). 

 But neither unproved assertions of this kind nor the more 

 refined attempts that have been made by others to bring the 

 phenomena of life and of dead matter under a common formula 

 touch the evidence for the atomic theory. The question as to 

 whether matter consists of elements capable of independent 

 motion is prior to and independent of the further questions as to 

 what these elements are, and whether they are alive or dead. 



The physicist, if he keeps to his business, asserts, as the bases 

 of the atomic theory, nothing more than that he who declines to 

 admit that matter con.sists of separate moving parts must regard 

 many of the simplest phenomena as irreconcilable and unintelli- 

 gible, in spite of the fact that means of reconciling them are 

 known to everybody, in spite of the fact that the reconciling 

 theory gives a general correlation of an enormous number of 

 phenomena in every branch of science, and that the outstanding 

 difficulties are connected, not so much with the fundamental 

 hypotheses that matter is composed of distinguishable entities 

 which are capable of separate motions as with the much more 

 difficult problem of what these entities are. 



On these grounds the physicist may believe that, though he 

 cannot handle or see them, the atoms and molecules are as real 

 as the ice crystals in a cirrus cloud which he cannot reach ; as 

 real as the unseen members of a meteoric swarm whose death- 

 glow is lost in the sunshine, or which sweep past us, unen- 

 tangled, in the night. 



If the confidence that his methods are weapons with which he 

 can fight his way to the truth were taken from the scientific 

 explorer, the paralysis which overcomes those who believe that 

 they are eng.aged in a hopeless task would fall upon him. 



Physiology has specially flourished since physiologists have 

 believed that it is possible to master the physics and chemistry 

 of the framework of living things, and since they have abandoned 

 the attitude of those who placed in the foreground the doctrine 

 of the vital force. To supporters of that doctrine the principle 

 of life was not a hidden directing power which could perhaps 

 whisper an order that the flood-gates of reservoirs of energy 

 should now be opened and now closed, and could, at the most, 

 work only under immutable conditions to which the living and 

 the dead must alike submit. On the contrary, their vital force 

 pervaded the organism in all its part?. It was an active and 

 energetic opponent of the laws of physics and chemistry. It 

 maintained its own existence not by obeying but by defying 

 them ; and though destined to be finally overcome in the separate 

 campaigns of which each individual living creature is the scene, 

 yet like some guerilla chieftain it was defeated here only to re- 

 appear there with unabated confidence and apparently un- 

 diminished force. 



This attitude of mind checked the adv.ance of knowledge. 

 Difficulty could be evaded by a verbal formula of explanation 

 which in fact explained nothing. If the mechanical, or physical, 

 or chemical causes of a phenomenon did not lie obviously upon 

 the surface, the investigator was tempted to forego the toi I of 

 searching for them below ; it was easier to say that the vital 

 force was the cause of the discrepancy, and that it was hopeless 

 to attempt to account for the action of a principle which was 

 incomprehensible in its nature. 



For the physicist the danger is no less serious, though it lies 



