466 



NATURE 



[June i, 191 i 



It commonly happens that tlie materials thus obtained 

 are, like the parent compound, explosive, and they are, 

 moreover, veiy sensitive to light. From a study of their 

 refractive power, it appears that in the aliphatic scries the 

 increment of refraction for the triazo-group is g-gi as 

 compared with 8-93 for bromine ; and whilst the atomic 

 dispersion of this halogen is 0-35, that attributable to the 

 triazo-group is 0-36. Moreover, the elevation of boiling 

 point produced by this complex lies between those due to 

 bromine and iodine, whilst the effect on the dissociation 

 constant of an aliphatic acid exceeds that of iodine, but 

 is less than that of bromine. Thus the physical evidence 

 supports those chemical characteristics which classify the 

 triazo-group as a complex radicle having a strong family 

 resemblance to the halogens. 



The changes undergone by triazo-compounds may be 

 referred to one of three main typts : — 



(i) Liberation of two nitrogen atoms in elemental form, 

 leaving the third attached to the carbon which originally 

 carried all three. This is exemplified by triazoantipyrine, 

 which passes spontaneously into a red azo-compound, 

 whilst in triazoacetone, triazocamphor, and triazoacetic 

 acid the change is accelerated by alkali. Sometimes this 

 type of alteration is violently explosive, as in the case of 

 triazoacetic azide. 



(2) Unfolding of the three-atom nitrogen ring into a 

 straight chain such as occurs in diazoaminobenzene or in 

 the cycloid tetrazole. Allylazoimide, for instance, a colour- 

 less liquid, changes spontaneously into an isomeric solid 

 which no longer contains the triazo-group, whilst hydrazoic 

 acid converts prussic and fulminic acids into tetrazole and 

 hydroxytetrazole respectively. 



(3) Complete removal of the triazo-group in the form of 

 hydrazoic acid. The simplest example of this change is 

 found in the behaviour of triazomethylamine derivatives, 

 •which liberate hydrozoic acid when treated with cold 

 water ; this happens also when triazotised carbon is 

 associated with a halogen, as in the case of triazoethylene 

 dibromide, but more generally this type of decomposition 

 requires the action of alkali. 



Although the preparation of chloroazoimide by Raschig 

 (1908) indicates the possibility of producing hexatomic 

 nitrogen by the union of two triazo-groups, this new form 

 of the element has not yet been realised. 



A NEW METffOD OF CHEMICAL ANALYSIS.^ 

 T HAVE had on several occasions the privilege of bring- 

 ing before the members of the Royal Institution some 

 of the results of the experiments on the positive rays on 

 which 1 have been engaged for the last few years. I 

 wish this evening to direct your attention to some applica- 

 tions of these to various chemical problems. 



The first application I shall consider is the use of these 

 rays to determine the nature of the gases present in a 

 vacuum tube, to show how they can be used to make a 

 chemical analysis of these gases — an analysis which, as 

 we shall see, will enable us to determine, not merely 

 whether an element, say, for example, oxygen, is present 

 in the tube, but will tell us in what form it occurs, 

 whether, for example, it is present in the atomic as well 

 as the molecular condition, and whether there are allo- 

 tropic modifications present, such as ozone, 0„ and other 

 still more complex aggregations. 



The method is as follows : the positive rays, after pass- 

 ing through a fine tube in the kathode, are exposed 

 simultaneously to magnetic and electric forces, the mag- 

 netic field being arranged so as to produce a vertical 

 deflection of the rays, while the electric field produces a 

 horizontal deflection. Thus, if when neither electric nor 

 magnetic fields are present, the rays strike a screen placed 

 at right angles to their direction at a point O, they will, 

 when both electric and magnetic forces are at work, strike 

 It at a point P, where the length of the vertical line PN 

 IS equal to the deflection produced bv the magnetic field, 

 and the horizontal line ON to that produced by the 

 electric field. ^ 



We know from the theory of the action of electric and 

 magnetic fields on moving electrified particles that 



Sir J^ J.' Thomson,' F:r!s" '^' ^°^"' '"^'""''°" °" ^"^^>> ^P"' ?• »>>' 



PN = A ' 



ON = B-i. 



NO. 2170, VOL. 86] 



where A and B are constants depending on the s: 

 of the magnetic and electric field* and the geometric;i . 

 of the tube, « ii the charge on the particle, m its mast, 

 and V its velocity. 



I'rom these relations we see that 

 m^\-^ ON 

 e B PX"* 



When these rays strike against a photographic plate, th- . 

 affect the plate at the point against which they 

 and thus when the plate is developed we have a pern 

 record of the deflections of the rays. The ni- ;1: 

 taking these photographs and the details of : ■ \;> r. 

 ment are described in my paper in The Vhinj^ophicai 

 Magazine, February. The values of A and B can be 

 determined accurately by the methods I have given in 

 previous papers, and hence if we measure on the photo- 

 graphs the values of ON and PN, we can determine th*: 

 value of mie. If we wish to compare the values of mjc 

 for two different rays, it is not necessary to determine A 

 and B ; all we have to do is to measure the values of 

 ON and PN, and thus the photograph alone gives us the 

 means of comparing the value of »a/c. 



Since for the same type of carrier m'e is constant, so 



PN* 

 that whatever may be the velocity -,^. is constant, and 



NO 

 therefore the locus of P, i.e. the curve traced on the 

 photographic plate by this carrier, is a parabola. The 

 reason we get a curve instead of a point is that the rays 

 are not all moving with the same velocity, and the slower 

 ones suffer greater deflection than the quicker ones. Each 

 type of carrier produces its own line on the plate, and 

 there are as many curves on the plates as there are kinds 

 of carriers ; from an inspection of the plates we can 

 find, not merely the number of kinds of carriers, but from 

 the dimensions of the curves we can at once determine the 

 atomic weight of the carrier, and thus determine its 

 nature. This is one of the great advantages of this 

 method. To illustrate this advantage, let us compare the 

 method with that of spectrum analysis. If the spectro- 

 scopist observes a line unknown to him in the spectrum 

 of a discharge tube, the most he can deduce without 

 further investigation is that there is scwne unknown sub- 

 stance present in the tube ; and even this would be 

 doubtful, as the new line might be due to some alteration 

 in the conditions of the discharge. But if we observe a 

 new curve in the positive-ray spectrum, all we have to do 

 is to measure the curve, and then we know the atomic 

 weight of the substance which produced it. To take an 

 example, I have photographed the positive-ray spectrum 

 for nitrogen prepared from the atmosphere and that for 

 nitrogen prepared from some nitrc^enous compounds, and 

 have found that the former contains a line ' which is not 

 in the latter, and that the value of mje for this line is 

 40 times that for the atom of hydrogen. We thus know 

 that atmospheric nitrogen contains an element of atomic 

 weight 40, which is not present in chemical nitrogen — this 

 element is, of course, argon. We might by ordinary spec- 

 trum analysis have found lines in the spectrum of atmo- 

 spheric nitrogen which are not in the spectrum of chemical 

 nitrogen, and might thus have suspected the presence of 

 another element ; but spectrum analysis could not tell us 

 anything about the nature of this element, whereas the 

 positive-ray spectrum at once gives us its atomic weight. 



The positive-ray method is even more delicate than that 

 of spectrum analysis, for by it we can detect the presence 

 of quantities of a foreign gas too minute to produce any 

 indication in the spectroscope. I have, for example, often 

 been able to detect the presence of helium by this method 

 when no indication of its presence could be detected by 

 spectroscope. 



Again, when a line in the positive-ray spectrum can be| 

 seen, the atomic weight of the carrier which produces il 

 can be determined with great accuracy. Though 

 method is only a few months old, it is even now suffi< 

 ently developed to determine with an accuracy of i 



1 As a matter of fact, there is a second, ver»- faint line for which ml* k i 

 about twenty times that for the atom of hydro.cen. This is probably due 10 

 an atom of argon with two electric charges. 



\ 



