September 12, 1884.] 



SCIENCE 



249 



The address of Sir William Thomson as presiding 

 officer of section A must be carefully read and stud- 

 ied to be appreciated. One or two of the ' steps 

 towards a kinetic theory of matter ' may be usefully 

 referred to. The as yet unsurmounted difficulty in 

 the kinetic theory of gases is the explanation of 

 what actually takes place during a molecular collis- 

 ion. It need hardly be said that physicists will not 

 be satisfied until more or less of the obscurity sur- 

 rounding this subject is dissipated. The mutual 

 action at the moment of collision has been generally 

 assumed to be repulsive by all who have written of 

 or contributed to the kinetic theory. Sir William 

 Thomson asks, May it not, after all, be attractive? 

 Under certain conditions it seems that the appear- 

 ance of repulsion may be the result of attraction. In 

 general, two molecules approaching each other with a 

 high velocity, assumed to be due to their attraction 

 for each other, will approach obliquely, as the 

 chances of a square ' hit ' will be exceedingly small ; 

 they will then dash past each other in sharply con- 

 cave curves around their centre of gravity, and fly 

 asunder again, something, indeed, after the fashion 

 of a comet passing around the sun. " A careless on- 

 looker," says Sir William Thomson, "might imagine 

 they had repelled one another." The idea that this 

 mutual action might be attractive rather than repul- 

 sive had been in his mind for thirty-five years, but 

 up to the preparation of this address he had never 

 made any thing of it. 



But, after all, the molecules must be infinitely small 

 in order that they may never come in actual contact, 

 so that we cannot evade the consideration of the 

 effects of these real impacts when they do occur. 

 Concerning these impacts, but two views seem to be 

 open to us ; the one is to imagine the molecule to be 

 a little elastic solid ; the other, to conceive it to be a 

 ' configuration of motion in a continuous all-pervad- 

 ing liquid.' It is hardly necessary to say, that, in 

 the opinion of Sir William Thomson, the latter must 

 be the final hypothesis upon which we may rest. 



But as a convenient intermediate station he sug- 

 gested the conception of an elastic molecule, out of 

 which we might not only construct a model of a gas, 

 but, with some satisfaction, by linking these mole- 

 cules together we might explain the elasticity of a 

 solid. In a paper presented to the Koyal Society of 

 Edinburgh in March, 1883, of which the title only 

 had been published, he had shown how an elastic 

 system may be constructed, composed entirely of 

 suitably disposed masses in motion. A system of 

 four gyrostatic masses connected together by links 

 was shown to possess all of the properties of an ordi- 

 nary elastic spring, although composed of matter in 

 itself entirely devoid of elasticity. By properly link- 

 ing great numbers of such gyrostatic systems together, 

 a model of an elastic solid results. Such a hypo- 

 thetical solid lends itself easily to the explanation 

 of such effects as the rotation of the plane of vibra- 

 tion of a wave transmitted through it, as in Fara- 

 day's celebrated experiment pf the rotation of the 

 plane of polarization of a ray of light in a magnetic 

 field. 



Sir William Thomson considered further the possi- 

 bility of discarding entirely the postulate of rigidity 

 in the materials under consideration, and showed how 

 a hydrokinetic model of matter might be constructed 

 in which all the effects of 'action at a distance' 

 might take place. By means of this the model of a 

 perfect gas might be produced, in which, however, 

 there still exists the difficulty of explaining the case 

 of actual impact of the particles. Some ingenious 

 suggestions were made in the way of surmounting 

 this difficulty; and the whole address was enriched by 

 the most delightful digressions on the part of the 

 author, during which the manuscript was neglected, 

 and the section was afforded the pleasure of following, 

 as best it could, the great physicist in his involuntary 

 excursions into this most interesting but little- 

 explored domain of physical science. 



Lord Kayleigh, in his presidential address, had re- 

 ferred at some length to recent investigations con- 

 cerning the theory of lubricants; and the section was 

 therefore in a favorable mood to listen to the first 

 regular paper on the programme, which was a theo- 

 retical consideration of that subject by Professor 

 Osborne Reynolds. 



The hitherto unrecognized results obtained by Mr. 

 Tower in his experiments were referred to, Mr. Rey- 

 nolds undertaking to show that they were in strict 

 accordance with our knowledge of the laws of motion 

 of viscous fluids. Mr. Tower had found, that when 

 the rotating journal with its box was immersed in a 

 bath of the lubricant, the resistance was not more 

 than one-tenth of its value in ordinary oiling, and that 

 the journal was less likely to heat at higher than at 

 lower speeds. By boring a hole through the top of 

 the box, it was found that the oil was forced through 

 with considerable velocity; and on attaching a press- 

 ure gauge, as high as two hundred pounds per square 

 inch was indicated. The oil appeared to be carried up 

 by the motion of the journal, and to form a film upon 

 which the box rested. Mr. Reynolds showed that 

 there would necessarily result a difference of pressure 

 on the two sides of the vertical line through the cen- 

 tre of gravity in the thin space between the box and 

 journal ; the maximum being on one side or the other, 

 according as the rotation is one way or the other. 

 Mr. Tower had found, that if, after running the jour- 

 nal for some time in one direction, a reversal were 

 made, great heating would result. Owing to the 

 difference of pressure above referred to, it was to be 

 expected that this would occur; as, undoubtedly, the 

 box and journal became adapted to each other for a 

 certain direction of running, and when a reversal was 

 made some time would elapse before a re-adaptation 

 would be completed. This would explain why a new 

 journal and box would always heat on first being run, 

 however perfect they may be. Mr. Tower had lik- 

 ened the operation to a stroking of the fibres of the 

 metal in one way by one direction of revolution, and 

 the reverse stroking at the early part of a reversed 

 motion; but this was not the true explanation, as 

 the resistance was evidently a shearing resistance, 

 the sliding of one layer of oil over the other. Sir 

 William Thomson, in commenting upon the paper, 



