82 



ADDRESS OF THE PRESIDENT OF THE BRITISH ASSOCIATION, 



[1852. 



the Nebuke have revealed to us the probable existence in the yet 

 more distant universe of forces with which we were previously 

 unaC'^uainted. The highest authorities in this most advanced of 

 all the sciences acknowledge themselves unable even to conjecture 

 the nature of the forces which have produced and maintain the 

 diverse, yet obviously systematic, arrangement of the hosts of 

 stars which constitute those few of the Spiral Nebula; which have 

 been hitherto examined. Hence the importance of increasing 

 our knowledge of the variety of forms in which the phenomena 

 present themselves, by a similar examination of the Southern 

 Heavens to that which Lord Rosse is accomplishing in the 

 Northern Heavens ; hence also, we may believe, in great measure 

 the devotion with which his Lordship has directed the unprece- 

 dented mstrumental power which he has created almost exclusively 

 to the observation of nebuL-c. But \,'hilst we cannot but admire 

 the steadiness of purpose with which an object regarded as of 

 paramount importance is undeviatiugly pureued, we can scarcely 

 forbear to covet at least an occasional glance at bodies which from 

 their o-ieater proximity have more intimate relations with our- 

 selves, and which, when viewed with so vast an increase of optical 

 power, may afford instruction of the highest value in many 

 branches of physical science. In our own satellite, for example, 

 we have the opportunity of studying the physical conformation 

 and superficial phenomena of a body composed, as we believe 

 mainly at least, of the same materials as those of our own globe, 

 but possessing neither atmosphei'e nor sea. When we reflect how 

 much of the surface of the earth consists of sedimentary deposits, 

 and consequently how large a portion of the whole field of geo- 

 logical research is occupied with strata which owe their principal 

 characteristics to the ocean in which they were deposited, we 

 cannot but anticipate many instructive lessons which may be 

 furnished by the points of contrast, as well as of resemblance, 

 which the surface of the moon, viewed through Lord Rosse's 

 telescope, may present to the best judgment we are able to form 

 of what the appearance of the earth would be if similarly viewed, 

 o ■ — with what may be more difficult perhaps to imagine — what 

 we may suppose the earth would appear if it could be stript of 

 its sedimentary strata which conceal from us for the most part the 

 traces of that internal action which has played so large a part in 

 mouldino- the great outlines of the ]^resent configuration of its 

 surface. It is underetood that Lord Rosse himself participates in 

 the wish that such an examination of the surface of the moon 

 should be made, — and, should the desire of the Association be 

 expressed to that effect, is willing to undertake it in coujunction 

 with one or two other gentlemen possessing the necessary physi- 

 cal and geological knowledge, it will be for the membei-s of the 

 Association to determine the fonn in which a Report on the 

 ' Physical Features of the Moon compared with those of the 

 Earth' may most appropriately be requested. 



The Mathematical and Physical Theories of Light have 

 afforded subjects for many interesting and profitable discussions 

 in Section A, and have usually had one day in the six specially 

 allotted to them. Those discussions will derive a more than 

 usual interest at this meeting from the remarkable discovery 

 recently made by Prof. Stokes, that under certain circumstances 

 a change is effected in the refrangibility of light, — and from the 

 advantacfe we possess in having amongst us on this occasion the 

 eminent mathematician and physicist by whom this most impor- 

 tant contribution to the science of physical optics has been made. 

 His researches took their origin fiom an unexplained phenomenon 

 discovered by Sir John Hei-schel, and communicated by him to 

 the Royal Society in 1845. A solution of sulphate of quinine 

 examined by transmitted light, and held between the eye and the 

 light, or between the eye and a white object, appears almost as 

 transparent and colourless as water ; but when viewed in certain 

 as]X'cts and under certain incidences of light, exhibits an extremely 

 vi\id and beautiful celestial blue colour. This colour was shown 



by Sii- John Herschel to result from the action of the strata 

 which the light first penetrates on entering the liquid ; and the 

 dispei-sion of light producing it was named by him epipolic dis- 

 pereion, from the circumstance that it takes place near the surface 

 by which the light enters. A beam of light having passed 

 through the solution was to all appearance the same as before its 

 entrance; nevertheless, it was found to have undergone some 

 mysterious modification, — for an epipolized beam of light — 

 meaning thereby a beam which had once been transmitted 

 through a quiniferous solution, and had experienced its dispersive 

 action — is incapable of further epipolic dispersion. In specula- 

 ting on the possible nature of epipohzed light. Prof. Stokes was 

 led to conclude that it could only be light which had been depri- 

 ved of certain invisible rays which in the process of dispersion 

 had changed their refrangibihty and had thereby become visible. 

 The truth of this supposition, novel and surprising as it at first 

 appeared, has been confirmed by a series of simple and perfectly 

 decisive experiments ; showing that it is in fact the chemical rays 

 of the spectrum, more refrangible than the violet, and invisible in 

 themselves, wliich produce the blue superficial light in the quini- 

 ferous solution. Prof. Stokes has traced this principle through 

 a great range of analogous phenomena, including those noticed 

 by Sir David Brewster in his papere on "Internal Dispereion ;" 

 and has distinguished between " cases of false internal dispersion" 

 or " opalescence," in which the luminous rays are simply reflected 

 from fine particles held in mechanical solution in the medium, 

 and those of " true internal dis|>ersion," or " fluorescence," as it is 

 termed by Prof. Stokes. By suitable methods of observation the 

 change of refrangibility was detected, as produced not only by 

 transparent fluids and solids, but also by opa^jue substances; and 

 the class of media exhibiting " fluorescence" was found to be very 

 large, consisting chiefly of organic substances, but comprehending, 

 though more rarely, some mineral bodies. The direct application 

 of the fact, as we now undei-sfaind it, to many highly interesting 

 and important purposes, is obvious almost on the firet announce- 

 ment. The facility with which the highly refrangible invisible 

 rays of the spectrum may be rendered visible by being passed 

 through a solution of sulphate of quinine or other sensitive 

 media, affords peculiar ad\antages for the study of those rays; 

 the fixed lines of the invisible part of the solar spectrum may now 

 be exhibited to our view at pleasure. The constancy with which 

 a particular mode of changing the refrangibility of light attaches 

 to a particular substance, exhibiting itself independently of the 

 admixture of other substances, supplies a new method of analysis 

 for organic compounds which may prove valuable in organic 

 chemistry. These and other applications of the facts as they are 

 now explained to us, will probably form subjects of notice in the 

 Chemical and Physical Sections; and a still higher interest may 

 be expected from the discussion of the principle itself, and of the 

 foundation on which it rests. A discovery of this nature cannot be 

 otherwise than extremely fertile in consequences, whether of direct 

 application, or by giving rise to sug'gestions branching out more 

 and more widely, and leading to trains of thought and experiment 

 which may confer additional value on the crigiual discovery by ren- 

 dering it but the first step in a siill more extensive generalization. 



Among the subjects of chemical inquiiy which may well deserve 

 the attention of a combination of philosophers, perhaps few could 

 more usefully occupy their joint labours than the revision of the 

 Equivalent Numbers of the Elementary Bodies. This is a task 

 which must necessarily require the co-operation of several properly 

 qualified individuals, if it be accomplished in anything like & 

 reasonable period of time. Most of the Numbei-s now im use 

 depend upon experiments performed by Berzelius, at a time when 

 the metho(.ls of research then known were inadequate, even in 

 such hands, to deteimine these constants with an accuracy suffi- 

 cient for the wants of science at the present day. So much has 

 this been felt to be the case, that many of the most accomplished 



