372 



NATURE 



\_August 1 8, 1887 



In all the foregoing instances we have a successful 

 correlation of the results of physical chemistry with those 

 obtained by the pure chemist in the deduction of constitu- 

 tional formulas. Many who withheld judgment, or even 

 condemned, when the chemist was his own witness, may 

 listen to him more favourably now that he is supported by 

 the independent testimony of the physicist. 



These investigations into the physical properties of 

 organic compounds are of relatively recent date. There 

 is little doubt that as they are extended new and impor- 

 tant laws will be deduced. Much is to be hoped from 

 thermo-chemistry, incoherent as many of its utterances as 

 yet are. By following the path of physico-chemical re- 

 search, chemists may even hope to arrive at a dynamical 

 representation of the molecule which shall be as much 

 more powerful as an instrument of research than the 

 present merely statical constitutional formula, as that is 

 more powerful than the empirical formula which preceded 

 it. It is hazardous to try to fix a limit to scientific 

 advance in any direction, but it is probable that the 

 modern constitutional formula represents the limit to what 

 purely chemical research can accomplish in determining 

 the constitution of matter. Much will still have to be 

 done by purely chemical research in working out the 

 details of the existing system : the constitution of the 

 more complex compounds will be ascertained on the lines 

 of our present formulae ; new valuable natural compounds 

 will be synthesized. But mere chemical reactions can 

 probably never settle questions of intramolecular 

 dynamics ; in these help must come from physical 

 chemistry. Moreover, the physical methods of research 

 supplement the chemical methods in one important 

 particular. By chemical methods we can never study 

 the molecule as it actually exists. Uur synthetical 

 methods give us information concerning the molecule 

 only at the moment of its formation ; our analytical 

 methods equally confine themselves to the moment of its 

 destruction. The physical methods supply this v/ant : 

 they enable us to study the existing compound. Of these 

 physical methods, one of the most promising, although 

 one of the most recent, is the optical method, which has 

 yielded results of the utmost importance both in inorganic 

 and in organic chemistry. The ray of light which passes 

 from the fixed star to the earth gives us information con- 

 cerning the composition of the atmosphere of the fixed 

 star ; and it is perhaps not too much to hope that the ray 

 of light which has threaded its way through and between 

 the molecules of a compound, and has been modified by 

 its contact with these, will, if properly interrogated, 

 furnish some information concerning the structure of 

 these molecules. Indeed, in the case of the rotation of 

 the plane of polarized light by organic liquids, of their 

 absorption spectra and their indices of refraction, this 

 information has in a measure been obtained. 



To sum up. The constitutional formula is not an 

 ultimate expression of the whole truth as regards 

 molecular structure. But it is certainly a very useful and 

 convenient symbolical expression of certain aspects of the 

 truth. We all hope that it may one day be superseded 

 by some higher and more complete generalization. But 

 it will be absorbed and assimilated, not rejected and 

 contradicted, by that generalization. Non oimiis morietur. 



THE YALE COLLEGE MEASUREMENT OF 

 THE PLEIADESy 



'T^HE Messrs. Repsold have established, and for the 

 -*• present seem likely to maintain, a practical 

 monopoly in the construction of heliometers. That com- 

 pleted by them for the Observatory of Yale College in 



^ " Determmation of the Relative positions of the Principal Stars in the 

 Group of the Pleiades." By William L. Elkin. Transactions of the 

 Astronomical Observatory of Yale University. Vol. I., Part I. (New 

 Haven : 1887.) 



1882 leaves so little to be desired as to show excellenc( 

 not to be the exclusive result of competition. In mer< 

 size it does not indeed take the highest rank ; its apertun 

 is of only 6 inches, while that of the Oxford heliomete: 

 is of 7^ ; but the perfection of the arrangement: 

 adapting it to the twofold function of equatoria 

 and micrometer, stamps it as a model not easy to be sur 

 passed. Steel has been almost exclusively used in thi 

 mounting. Recommended as the material for the objective 

 cell by its quality of changing volume under variations 

 temperature ne.7\.r\y pari passu with glass, its employmen 

 was extended to the telescope-tube and other portions 

 the mechanism. The optical part of the work was don^ 

 by Merz, Alvan Clark having declined the responsibilit; 

 of dividing the object-lens. Its segments are separabli 

 to the extent of 2°, and through the contrivance 

 cylindrical slides (originally suggested by Bessel) perfec 

 definition is preserved in all positions, giving a rang 

 of accurate measurement Just six times that with a fila 

 micrometer. (Gill, " Encyc. Brit." vol. xvi, p. 253 

 Fischer, Sirius, vol. xvii. p. 145.) 



This beautiful engine of research was in 1883 place( 

 in the already practised and skilful hands of Dr. Elkir 

 He lost no time in fixing upon a task suited both to tes 

 the powers of the new instrument and to employ them t' 

 the highest advantage. 



The stars of the Pleiades have, from the earliest time' 

 attracted the special notice of observers, whether savag 

 or civilized. Hence, on the one hand, their prominenc 

 in stellar mythology all over the world ; on the other, thei 

 unique interest for purposes of scientific study and com 

 parison. They constitute an undoubted cluster ; that is t 

 say, they are really, and not simply in appearance, groupe 

 together in space, so as to fall under the sway of prevailin 

 mutual influences. And since there is, perhaps, no othe 

 stellar cluster so near the sun, the chance of perceptibl 

 displacements among them in a moderate lapse of tim 

 is greater than in any other similar case. Authentic dat 

 regarding them, besides, have now been so long garnere 

 that their fruit may confidently be expected at least t 

 begin to ripen. 



Dr. Elkin determined, accordingly, to repeat the surve 

 of the Pleiades executed by Bessel at Konigsberg durin. 

 about twelve years pi^evious to 1841. Wolf and Pritchar 

 had, it is true, been beforehand with him ; but the wid 

 scattering of the grouped stars puts the filar micromete 

 at a disadvantage in measuring them, producing minut 

 errors which the arduous conditions of the problei: 

 render of serious account. The heliometer, there can b 

 no doubt, is the special instrument for the purpose, an( 

 it was, moreover, that employed by Bessel ; so that th 

 Konigsberg and Yale results are comparable in a stride 

 sense than any others so far obtained 



One of Besiel's fifty-three stars was omitted by Di 

 Elkin as too faint for accurate determination. He added 

 however, seventeen stars from the Bonn Durchimtsfefutii^ 

 so that his list comprised sixty-nine, down to 9'2 magni 

 tude. Two independent triangulations were executed b 

 him in 1884-85. For the first, four stars situated nea 

 the outskirts of the group, and marking the angles of ; 

 quadrilateral by which it was inclosed, were chosen a 

 reference-points. The second rested upon measure 

 of distance and position-angle outward from Alcyone 

 (r; Tauri). Thus, two wholly unconnected sets of posi 

 tions were secured, the close accordance of which testifie( 

 strongly to the high quality of the entire work. The; 

 were combined, with nearly equal weights, in the fina 

 results. A fresh reduction of the Konigsberg observa 

 tions, necessitated by recent improvements in the valui 

 of some of the corrections employed, was the preliminar; 

 to their comparison with those made, after an interval 

 forty-five years, at Yale College. The conclusions thu 

 laboriously arrived at are not devoid of significance, anc 

 , appear perfectly secure, so far as they go. 



