396 



NATURE 



[May 27, 1920 



This method has been used by M. Nicolardot 

 in the chemical laboratory of the Technical Section, 

 French Artillery, and according- to Gramont it 

 gives very satisfactory results in the control of 

 chemical analyses. The spectrograph has also 



510 



Vv*^vs:\_icr«i<a-rvH 



/V^ 



Fig. 3. — Curves plotted from results obtained in examining dyes wii 

 spectrophotometer. 



been for some time in use at the Bureau of 

 Standards, Washington, for determining small 

 quantities of impurities in tin and in the analysis 

 of steel, especially as reg-ards chromium and 

 titanium. For estimating small quantities of 

 elements such as niobium and molybdenum, the 

 NO. 2639, VOL. 105] 



spectfographic method is, indeed, stated to be 

 more trustworthy than chemical analysis. 



Another analytical method for metals has been 

 described, depending^ upon a difference in volatility 

 of the elements present. The authors (Hill and 

 Luckey) use the "arc" discharge, and measure 

 the time required for a given line to disappear 

 when a known weight of the material is burned 

 in the crater of the arc. Though this process is 

 of limited application, it can be used for the 

 estimation of lead in copper, within a range of 

 0*004 to o'2i6 per cent., with the accuracy neces- 

 sary for the work of a copper refinery. 



The spectrophotometer, too, is now finding 

 technical application, especially in connection with 

 dyes. As the study of absorption-spectra has pro- 

 gressed, means for making the observations more 

 strictly quantitative have developed also. The 

 possibility of measuring the absorption of a sub- 

 stance for light of each wave-length is, in fact, 

 an important addition to the resources of the 

 organic chemist in dealing with certain technical 

 problems. 



The apparatus employed is a spectrograph or 

 spectrometer combined with a suitable photometer 

 such as the " Nutting " instrument. It is used in 

 the control and analysis of dyes, the chemical 

 testing of which is often a difficult matter. In 

 pre-war days purchasers of dyes were very much 

 at the mercy of foreign dye-makers as regards the 

 quality and strength of dyes sent to them. 

 Spectrophotometry can now be employed to safe- 

 guard the interests of the user in this respect. 

 For example, a solution of known strength can 

 be prepared from a trustworthy specimen of dye, 

 and its colour-density determined for a series of 

 wave-lengths by the spectrophotometer; a curve 

 plotted from the results can then be kept as a 

 permanent reference with which future supplies 

 can be compared. Similarly the colour-producing 

 value of a dye with various illuminants may be 

 assessed by means of the instrument. Dyeing tests 

 can be quantitatively controlled by comparing the 

 intensity of reflected light from the dyestuff in 

 each part of the spectrum with that of light 

 reflected from a white surface. 



The proportion of diluent substance added to a 

 dye, or of two dyes in a mixture, may also be de- 

 termined by reference to standard curves. Thus in 

 the subjoined diagram (Fig. 3), A and B denote 

 such curves for known strengths of eosin and ery- 

 throsin respectively, and C is a curve given 

 by a mixture of the two substances, in un- 

 known proportions. By taking ordinates 

 for two suitable wave-lengths, two equations 

 can be formulated, from which the propor- 

 the tions of the two dyestuffs in the mixture 

 are calculated. From these examples 

 the value, actual and potential, of the instru- 

 ment to the dyeing industry will readily be 

 understood. 



Of other technical uses to which special instru- 

 ments are applied, a brief mention must suffice. 

 Thus in the iron and steel industry certain rapid 



