440 REPORT— 18 82. 



retained liis opinion. But the possibility of further analysis of light of definite 

 refrangihility (except by polarisation) is almost irreconcilable with the wave 

 theory, which on the strongest grounds had been already accepted by most of 

 Brewster's contemporaries ; and in consequence his results, though urgently pressed, 

 failed to convince the scientitic world. Further experiment has fully justified 

 this scepticism, and in the hands of Airy, Helmholtz, and others, has shown that 

 the phenomena by which Brewster was misled can be explained by tbe unrecognised 

 intrusion of ditl'used light. The anomalies disappear when sufficient precaution is 

 taken that the refrangihility of the light observed shall really be definite. 



On similar grounds undulationists early arrived at the conviction that physically 

 light and invisible radiant heat are both vibrations of the same kind, differing 

 merely in wave-length ; but this view appears to have been accepted slowly, and 

 almost reluctantly, by the experimental school. 



ANTien the facts which appear to conflict with theory are well-defined and lend 

 themselves easily to experiment and repetition, there ought to be no great delay in 

 arriving at a judgment. Either the theory is upset, or the observations, if not 

 altogether faulty, are found susceptible of another interpretation. The difficulty is 

 greatest when the necessary conditions are uncertain, and their fulfilment rare and 

 uncontrollable. In many such cases an attitude of reserve, in expectation of further 

 evidence, is the only wise one. Premature judgments err perhaps as much on 

 one side as on the other. Certainly in the past many extraordinar}' observations 

 have met with an excessive incredulity. I may instance the fire-balls which some- 

 times occm- dui'ing violent thunderstorms. AVhen the telephone was first invented, 

 the early reports of its performances were discredited by many on quite insufficient 

 grounds. 



It would be interesting, but too difficult and delicate a task, to enumerate and 

 examine the various important questions which remain still undecided from the 

 opposition of direct and indirect evidence. Merely as illustrations I will mention 

 one or two in which I happen to have been interested. It has been sought to 

 remedy the inconvenience caused by excessive reverberation of sound in cathedrals 

 and other large unfurnished buildings by stretching wires overhead from one wall 

 to another. In some cases no difference has been perceived, but in others it is 

 thought that advantage has been gained. From a theoretical point of view it is 

 difficult to believe that the wires could be of service. It is known that the 

 vibrations of a wire do not communicate themselves in any appreciable degree 

 directly to the air, but require the intervention of a sounding-board, from which 

 we may infer that vibrations in the air would not readily communicate themselves 

 to stretched wires. It seems more likely that the advantage supposed to have been 

 gained in a few cases is imaginary than that the wires should really have played 

 the part attributed to them. 



The other subject on which, though with diffidence, I should like to make a 

 remark or two, is that of Front's law, according to which the atomic weights 

 of the elements, or at any rate of many of them, stand in simple relation to that 

 of hydrogen. Some chemists ha^•e reprobated strongly the importation of 

 a priori views into the consideration of the question, and maintain that the 

 only numbers worthv of recognition are the immediate results of experiment. 

 Othej's, more impressed by the argument that the close appro.ximations to simple 

 numbers cannot be merely fortuitous, and more alive to tlie inevitable imperfec- 

 tions of our measurements, consider that the experimental evidence against the 

 simple numbers is of a very slender character, balanced, if not outweighed, by the 

 a priori argument in favour of simplicity. The subject is eminently one for further 

 experiment; and as it is now engaging the attention of chemists, we may look for- 

 ward to the settlement of the question by the present generation. The time has 

 perhaps come when a re-determination of the densities of the principal gases may 

 be desirable — an undertaldng for which I have made some preparations. 



If there is any truth in the views that I have been endeavouring to impress, 

 our meetings in this Section are amply justified. If the progress of science demands 

 the comparison of evidence drawn from different sources, and fullj' appreciated only 

 by minds of different orders, what may we not gain from the opportunities here given 



