March 19, 1885] 



NA TURE 



knowledge we must set the more systematic training to 

 which students are subjected before they are expected to 

 take an independent line ; secondly, the new methods of 

 treatment and new points of view which now succeed one 

 another more rapidly than at any previous time ; and, 

 thirdly, the very greatly increased facilities for research on 

 the spot in foreign countries. When it is remembered how 

 many of our most prominent men started their careers as 

 travellers, the importance of the third of the above con- 

 siderations will be valued as it ought to be. Those who 

 are best able to appreciate the position of anatomical and 

 physiological botany would probably be the first to agree 

 that the opportunities for research in these branches, 

 either in foreign lands or at home, are, at the present 

 moment, better than they have been at any former period 

 in the history of the science. If the botanical students 

 of the present day content themselves with devoting their 

 time and energy to working out small and uninteresting 

 details, it is their own poverty of imagination and want 

 of enterprise that are to blame. F. O. B. 



MOLECULAR DYNAMICS 1 

 T HAVE placed the three titles above this article not 

 *■ because I intend to deal with more than the last, but 

 because they all deal with the same matter, and show how 

 much the author's attention was directed to the subject 

 during his three months' sojourn in America. The 

 audience at the Baltimore lectures consisted chiefly of 

 American professors, and a few English men of science 

 attended a larger or smaller number of the lectures. 



Speculation was rife as to the probable character of the 

 lectures, and there was a general feeling that vortex 

 motion would be largely dealt with. This, however, was 

 not so. The course of twenty lectures was confined to 

 the wave theory of light, largely dealing with the diffi- 

 culties of that theory. The published lectures are not 

 printed, but " jelligraphed," as Sir William Thomson 

 would say. The number of copies is extremely limited, 

 and are of unique interest, being reproduced from the 

 short-hand notes taken at the lectures. Every one who 

 knows how suggestive Sir William's lectures are and how 

 fertile his mind is in bringing illustrative digressions to 

 bear on the topic in hand, will expect these verbatim 

 notes to be a rare treasure. Nor will he be disappointed. 

 Mr. Hathaway, the reporter, has the unusual combination 

 of being an expert stenographist, a skilful mathematician, 

 and a clear and distinct caligraphist. His notes contain 

 numerous errors, such as are unavoidable in such an 

 undertaking, but, viewed as a whole, his work is almost a 

 marvel. 



The lectures treated of three branches ot the subject : 

 (1) the propagation of a disturbance through an elastic 

 medium ; (2) the character of molecular vibration ; and 

 (3) the influence of molecules on the propagation of waves. 

 Each lecture generally dealt with two of these branches, 

 and between the two parts of the lecture Sir William 

 went among his audience and had some conversation 

 with them. It was ever his object to discard the profes- 

 sorial attitude and give his lectures the aspect of confer- 

 ences. Discussion did not end in the lecture-room, and 

 the three weeks at Baltimore were like one long confer- 

 ence guided by the master mind. It is not surprising 

 that at the end of that time there was a genuine feeling 

 of sadness at parting on the part of teacher and taught 

 alike. 



The part of the lectures dealing with the propagation 

 of an elastic disturbance could not be expected to contain 



1 "On Molecules," the Presidential Address to Section A of the British 

 Association, August, 1884. by Sir William Thorns 



"The Wave Theory of Light," a Lecture delivered by Sir William 

 Thomson at Philadelphia on Sept. 29, 1384, published in Nature, vol. xxxi. 



' Lecti 



Hopkins 



Thomson, Johns 



much novelty, but it was treated in so novel a manner 

 and from so purely a physical point of view, that it could 

 not but be instructive. Many of the old supporters of the 

 theory dealt with it purely from » mathematical point of 

 view. They treated the problem as a mathematical exer- 

 cise, and did not hesitate to make unwarranted assump- 

 tions to produce pretty formulas or simple solutions. 

 Even such men as Weber (in his " Theory of Magnetism ") 

 and Green (in his " Wave Theory ") have been guilty of 

 this practice. Sir William Thomson never made any but 

 physical assumptions, and these were made for reasons 

 given. Rather than make a meaningless mathematical 

 assumption he would prefer to burden his formulas with 

 undetermined quantities, and even, if unable to reach the 

 final solution, would rejoice in the richness of the 

 formulas, which showed a potentiality of overcoming 

 many difficulties. He does not always commend Ran- 

 kine's mathematics, but he says this for him at p. 185 : 

 " Rankine did a great deal to cure the mathematical 

 disease of asphasia from which we suffered so long. 

 Faraday did most. The old mathematicians used neither 

 diagrams to help people to understand their work, nor 

 words to express their ideas. It was formulas, and 

 formulas alone. Faraday was a great reformer in that 

 respect with his language of ' lines of force,' &c. Rankine 

 was splendid in his vigour and in the grandeur of his 

 Greek derivations." This refers to Rankine's nomen- 

 clature of different kinds of moduluses and their recipro- 

 cals — e.g. plagiotatic, thlipsinomic, &c. 



The first lecture is a summary of what is to come, and 

 is partly historical. The difficulties in the way of accept- 

 ing the wave theory of light are clearly pointed out. 

 These are four in number. 



First Difficulty : Dispersion. — The difficulty is to 

 explain how velocity of propagation depends on period 

 of vibration. Two explanations have been offered, that 

 of Cauchy and that of Helmholtz. He does not delay 

 much with Cauchy, who ascribed it to heterogeneousness. 

 He prefers Helmholtz, who ascribes it to a compound 

 structure of material molecules, which gives them a 

 natural period of vibration. The one explanation has 

 relation to wave-length, the other to period of vibration. 

 The latter, he thinks, falls in better with results of spec- 

 trum analysis, &c. A great portion of the lectures is 

 devoted to expanding the notion of Helmholtz. The 

 space occupied by a molecule must be filled with a sub- 

 stance differing from the ether either in rigidity or in 

 density, or both. Lord Rayleigh has taken in hand 

 this question in his researches on blue sky, and it seems 

 that a variation of density is the principal or only effective 

 cause. With respect to the new (Helmholtz-Thomson) 

 spring and shell molecule, he says, " It seems to me that 

 there must be something in this, that this, as a symbol, 

 is certainly not an hypothesis, but a certainty." 



Second Difficulty : tlic Ether. — He makes short work 

 of the difficulty of reconciling almost perfect rigidity with 

 almost perfect mobility. It is merely a matter of time. 

 You can make a tuning-fork of Burgundy pitch when the 

 period is a small fraction of a second, but a bullet will 

 pass clean through several inches of it in six months. The 

 ether may be highly elastic for vibrations executed in the 

 100 or 1600 million millionth of a second, but highly 

 mobile to bodies going through it at the rate of twenty 

 miles a second. 



Third Difficulty: Refraction and Reflection. — Theo- 

 retical equations agree qualitatively with facts, but there 

 are serious discordances when we come to quantitative 

 measurements. Especially is this the case in the com- 

 pleteness of extinction of the ray polarised by reflection. 



Fourth Difficulty: Double Refraction. — It is found that 

 when the medium is displaced during wave-propagation 

 in a double refracting crystal, the return force must 

 depend on the direction of vibration, not on the plane of 

 distortion, as all elastic theories indicate. Rankine and 



