352 



NATURE 



[Feb. 



diameter, and that as the speciniens decrease in length the con- 

 traction of area at the point of fracture decreases also ; and, in 

 consequence, the tensile strength increases when reckoned on 

 the original area, and decreases when reckoned on the fractured 

 area. The elongation in percentage of the original length is 

 also very much increased in the shorter specimens, owing to the 

 fact that the greater part of the elongation then takes place 

 much nearer to the point of fracture, instead of being more 

 equally distributed, as it is along the length of the longer bars. 



A paper was read by Mr. A. A. Langley, descriptive of a 

 hydraulic buffer-stop for railways, the chief advantages of which 

 are absence of recoil after collision, continuous uniform resist- 

 ance for bringing a train to rest, and absence of shock or break- 

 age either in train or in buffer. The chief feature is the appli- 

 cation of hydraulic resistance by the use of pistons working in 

 horizontal cylinders filled «ith water, and fixed in line with the 

 buffers of the rolling-s'ock. 



THE VALUE OF THE REFRACTION GONIO- 

 METER IN CHEMICAL WORK^ 



TMPORTANT advances in chemistry have often been the 

 result of new methods of research, and these have generally 

 involved the use of new apparatus. The introluction of the 

 balance made the greatest of revolutions in chemistry ; but the 

 thermometer, the blowpipe, the polariscope, and the spectro- 

 scope in its multifarious applications may also be cited as 

 examples. 



My object is to speak of the refraction goniometer or spectro- 

 meter, by v/hich the refraction and dispersion of bodies can be 

 measured. The construction of this instrument, and the use of 

 it, which ought to be a part of the regular training of chemical 

 students, may be learned from many works on physics, but it is 

 very fully described in Glazebrook's "Optics," together with 

 the manner in which the angular measurements are reduced to 

 refractive indices. By means of this instrument the index of 

 refraction is easily obtained for liquid bodies ; solids or gases 

 require a more complicated ajoparatus, but those of them which 

 can be readily dissolved in any liquid can be determined from 

 their solutions. 



The index of refraction du) of a body is a definite physical 

 property, like its boiling-point, specific gravity, or solubility, 

 and ought to form part of our knowledge of any substance. 1 

 have generally determined it for the line A of the spectrum of 

 the sun ; but Continental observers have usually adopted the 

 red line of hydrogen, which is coincident with the solar line C. 



The length of the spectrum — that is, the difterence between 

 the indices of refraction of extreme rays, say the lines H and A, 

 which may be taken as the measure of dispersion — is another 

 physical property, and an equally im|3ortant one. 



If the index of refraction and the dispersion of a substance be 

 accurately known, we have a double test of the purity of any 

 specimen that may have to be examined. 



As, however, the refraction varies with temperature and other 

 circumstances, it is better to deal with the specific refraction, 



that is, the index, minus unity, divided by the density ( ^ V 



This is little, if at all, affected by pressure, heat, change of 

 aggregate condition, mixture, solution, or, generally speaking, 

 by chemical combination. Thus the specific refraction of water 

 under different circumstances has been determined as follows, 

 the observations being reduced for the line A of the spectrum : — 



Water 



Liquid, at i° C. 

 at48°C. 



Solid 



Gas 



Mixture with alcohol ... 



Water of crystallisation 



Specific refractic 



... 0-329 



... 0329 



■■■ 0-331 = 



... 0-324 



... 0-3308 



... 0-330* 



... 0-327 ii 



The identity of the specific refraction of a body in the solid 

 state or in solution has been frequently proved ; the last instances 



t Communicated to Section E of the British Association, tat the Aberdeen 

 meeting, September 1885. 



2 Rausch. ^ Landolt ; mean of three observations. 



4 In ammonia alum ; Charles Soret. 



5 In double sulphates; Topsoe and Christiansen. 



determined were as follows, the observations on the crystals being 

 made by M. Soret, and on the solutions by myself: — 



Substance Solution Crystallisation 



Ammonia alum 0-2780 ... 0-2784 



Soda alum 0-2613 ... 0-2604 



For the purpose of calculation, however, it is more conve- 

 nient to adopt what Landolt denominated the refraction equiva- 

 lent, that is, the specific refraction multiplied by the atomic weight 

 ( P ^~ ' )■ The refraction equivalent of water may be taken, 



therefore, as 0-3295 X 18 = 5-93. Of this, the two atoms of 

 hydrogen may be assu ned from observations on other bodies to 

 represent 2-6, leaving for the atom of o.xygen 3'33. 



The specific refraction and dispersion of a body not merely 

 gives an indication as to its purity or otherwise, but tells the 

 quantity of the substance with which it may be mixed, if that 

 substance is known. Thus Landolt has applied it to the quan- 

 titative analysis of mixtures, and gives examples, such as ethyl 

 alcohol and fousel oil, ethyl alcohol and ether. I have applied 

 it myself in the estimation of carbolic acid in disinfecting pow- 

 ders, by dissolving the acid out in a known quantity of alcohol, 

 and determining the refraction and density of the solution. 



In chemical investigations among organic compounds the 

 determination of the specific refraction of the products is very 

 valuable. Thus, in a recent investigation on the action of the 

 copper-zinc couple on bromide of benzyl by Mr. Tribe and 

 myself, there were three different ways in which it led us to 

 results which we should not otherwise have arrived at. 



(i) The viscid mass which resulted from the action appeared 

 very unpromising, but, on examining it with the prism, its specific 

 refraction and dispersion were so high that we determined to 

 purify it, and this led to the discovery of the new hydrocarbon 

 benzylene. 



(2) When the reaction was performed in the presence of 

 alcohol, it seemed probable that toluene would be produced ; 

 but the liquid, when heated, distilled off at 78°, which is the 

 boiling-point of pure alcohol. Instead, however, of throwing 

 the distillate away, it was examined in a hollow prism, and seen 

 at once to be something very different. Indeed, the increased 

 refraction and dispersion led to the belief that one-fifth of it 

 was toluene, though that boils at 110°. On adding water, a 

 liquid separated, which was proved to be toluene by its boiUng- 

 point and density, as well as by its specific refraction and dis- 

 persion, 0-5604 and 0-0474, agreeing sufficiently well with the 

 known figures. 



(3) On another occasion, among products of fractional dis- 

 tillation was a liquid which had too small a refraction to allow 

 of its being considered a hydrocarbon. It was suspected that 

 the lo,v refraction might be due to oxygen or bromine ; and this 

 led to a further examination and the discovery that the liquid 

 contained a new bromine compound. 



But another important application of these physical properties 

 is to the elucidation of the chemical structure of various bodies. 

 A very large amount of information as to specific refraction is 

 now at our disposal through the labours of different experi- 

 menters, not only in this country, but in Germany, Italy, Russia, 

 Holland, and Sweden ; and the whole course of recent investi- 

 gations goes to show (i) that the specific refraction of a body 

 depends essentially on its ultimate atomic constitution ; but (2) 

 that this is modified in certain definite ways by the molectilar 

 arrangement or structure. Thus, to t;\ke an instance : sugar, 

 CioHo.iOii, whether crystallised or dissolved, has the refraction 

 equivalent of 119-3. Ifi however, we take each atom of carbon 

 at 5-0, and water at 5-93, we should obtain the figure 1 25 -I as 

 the calculated value. The discrepancy is far too great to be 

 attributed to errors of experiment, and points to the fact that 

 sugar is not, strictly speaking, a carbohydrate ; that really it does 

 not contain water, but that the hydrogen and oxygen are other- 

 wise arranged, as chemists have concluded on other grounds. 

 The oxygen in all hydroxyl compounds is 2-8, according to 

 Briihl, and this woidd give for sugar the theoretical value II9'5> 

 which is almo.st identical with the experimental number. 



In a similar way it has been very fully substantiated that 

 carbon, whenever it is in the condition which is termed " double- 

 linked," has the value, not of 5-0, but of about 6-i. Hence in 

 any compound the constitution of which is doubtful, we can tell 

 how many carbon atoms are in this condition. Thus, to take 

 terpene, CjoHjg. This has been the subject of much discus^jon 

 among chemists, one considering it to have one pair of carbon 



