140 
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| DECEMBER 6, 1906 
direction. These remaining circularly polarised rays can- | optical phenomena.’ He does not deal with the electrons 
not be extinguished by a Nicol. 
The brilliant yellow spot which appears on the screen 
as soon as the current is put on is due to such rays. The 
explanation of this experiment is not complete, however, 
at least not for denser vapours. The Faraday rotation of 
the plane of polarisation then plays a part, as we shall see 
further on. 
The magnetisation of the spectral lines allows us to 
determine whether positive or negative electrons are 
vibrating in a flame. From the phenomena in the direc- 
tion of the lines of force, it follows that in a luminous gas 
the negative electrons give rise to all vibrations. It does 
not follow, however, that the luminous molecules have a 
negative charge. On the contrary, the researches of 
Lenard and Stark show that at least part of the luminous 
spectra is emitted by positively charged atoms. 
When a line is split up into a triplet, we can, by 
measuring the amount of the effect, find out how much 
matter is loaded with the revolving electron, or, in other 
words, we can determine the ratio of the charge e to the 
mass m of the electron. In this manner I have made the 
first determination of this notable number e/m, and found 
it of the order of magnitude of 10’ electromagnetic units 
per gram.’ The most accurate measurements of the present 
time for different spectral lines yield values ranging 
between 1-4 and 1-8 by 10’. This number is about 1500 
times the corresponding number for hydrogen as deduced 
from the phenomena of electrolysis. 
We must, then, conclude that at least a majority of 
spectral lines is due to the vibrations of the negative 
electron. This conclusion is not only valid for incan- 
descent sodium or mercury. All elements which can give 
colour to a flame or which can be evaporated in a spark 
show the magnetisation of the spectral lines, and hence 
in all elements these negative electrons are present 
Independent experimental evidence for the existence of 
electrons has been derived from the study of the kathode 
rays in a vacuum tube. The discontinuous structure of 
electricity was also proved by other phenomena, and in 
this way physicists were led by purely experimental methods 
to the negatively charged corpuscle of J. J. Thomson, 
1500 times smaller than the hydrogen atom, in full accord- 
ance with the electron necessitated for the explanation of 
the magnetisation of the spectral lines. 
All fundamental characteristics of the magnetic resolu- 
tion of the spectral lines were then explained, and the 
truth of the explanation proved beyond the possibility of 
doubt. More detailed knowledge of the effect has been 
greatly extended by a whole series of investigators, 
especially by Becquerel, Cornu, Cotton, Michelson, Kent, 
Konig, Righi, Reese, Runge and Paschen, and in this 
country by Gray, Preston, Lodge, Lord Blythswood, and 
others, and from the theoretical side by Larmor, Fitzgerald, 
Jeans, and J. J. Thomson. 
Not all spectral lines are tripled; some are split up into 
quartets, others into sextets. The lines D, and D, in 
strong fields are an example. The whole of such a system 
of lines is, even in the strongest fields, confined to the 
space of one-sixth of the distance of the sodium lines. In 
some cases still more complicated subdivisions have been 
observed, especially by Michelson. In such cases the simple 
electromagnetic model of a molecule emitting light is in- 
sufficient. We shall return to this subject afterwards, and 
first proceed to a discussion of phenomena accompanying 
the inverse effect. 
This investigation, which I carried out in Amsterdam 
together with my pupils, Drs. Hallo and Geest, was sug- 
gested by a theoretical investigation by Prof. Voigt, of 
Gottingen. Lorentz’s theory relates to one single vibrating 
particle, and can only be applied to substances of very 
small density which emit very narrow spectral lines. With 
greater density, and therefore broader spectral lines, the 
mutual influence of the molecules must be taken into 
account. It seems, however, that a theory of emission 
of a system of reciprocally reacting molecules is rather 
difficult. In the case of absorption the problem is easier, 
and: is considered by Prof. Voigt in his theory of magneto- 
1 Zeeman, Verslagen Kon. 
§ 23. 
Akademie, Amsterdam, November, 1896, 
NO. 1936, VOL. 75 | 
directly, but adds suitable new terms to the equations of 
motion in an absorbing medium. His method establishes 
a connection between the rotation of the plane of polar- 
isation and the resolution of the spectral lines, a connec- 
tion almost simultaneously pointed out by Fitzgerald. 
This also led to an interesting result, until then missed 
by the electronic theory, namely, rotation of the plane of 
polarisation close to an absorption band. 
(To be continued.) 
MINERAL RESOURCES OF THE UNITED 
STATES. 
{ROM time to time we have directed attention to the 
variety and magnitude of the work being carried on 
by the United States Geological Survey. Of all the work 
undertaken, none is of greater value to the general public 
than that of the division of mining and. mineral statistics 
under the able direction of Dr. David T. Day, whose 
masterly report (Washington, 1905) on the mineral re- 
sources of the United States for 1904 has recently been 
issued. The volume covers 1264 closely printed pages, and 
in arrangement and scope is similar to the twenty pre- 
ceding annual reports of the series. Each chapter is a 
census of the productive features of the industry under 
discussion from the pens of statistical experts. The figures 
dealt with are stupendous. In 1904, for the fifth time, the 
total value of the mineral production of the United States 
exceeded the sum of 200,000,000l., and it is curious to note 
that the value of non-metallic minerals exceeded that of 
metallic minerals by 41,000,0001. Iron ore and coal are 
the most important of the mineral products. Statistics of 
the production of moulding sand were collected for the 
first time in 1904. Another novelty is a report, written 
by Mr. H. H. Hindshaw, directing attention to the occur- 
rence of peat in the United States and to its possibilities 
as a source of fuel. Appended to the report is a useful 
bibliography of the subject. Tin ore was produced com- 
mercially, though in small quantities, in South Carolina, 
South Dakota, and Alaska. 
There is a constant increase in the demand for such 
abrasives as corundum’ and emery, and consequently the 
publication of Bulletin No. 269 (Washington, 1906), on 
corundum and its occurrence and distribution in the United 
States, by Mr. J. H. Pratt, is a welcome addition to 
technical literature. It is an enlarged edition of Bulletin 
No. 180, published in 1901, and has grown from 98 pages 
to 175 pages. It is admirably illustrated with eighteen 
plates, and constitutes a complete monograph on the three 
varieties, sapphire or gem corundum, corundum, and 
emery. All the occurrences in the United States are de- 
scribed, and particulars/ re added regarding the distribu- 
tion of corundum in other ~untries, the methods of mining 
and cleaning, and the uses of conandum. In conclusion, 
some useful suggestions to prospectors for chromium are 
given. 
The most remarkable outcome of the recent work of the 
mining, division is the extensive series of tests of the various 
coals found in the United States. These are recorded in 
three bulky quarto volumes forming Professional Paper 
No. 48 (Washington, 1906), and covering 1492 pages. The 
work was carried out at the coal-testing plant of the United 
States Geological Survey at the St. Louis Exhibition under 
the direction of a committee consisting of Messrs. Edward 
W. Parker, Joseph A. Holmes, and Marius R. Campbell. 
The first volume describes the field work, classification of 
coals, and chemical work; the second deals with boiler 
tests; and the third with producer-gas, coking, briquetting, 
and washing tests. The results are of far-reaching import- 
ance in the solution of the fuel and power problems upon 
which the varied industries of the United States depend. 
Most of the American bituminous coals and lignites can, 
it was found, be used as a source of power in a gas- 
producing plant, the power efficiency of bituminous coals 
when thus used being two and a half times greater than 
1 Voigt, Annalen der Physik, Bd. 67, S. 345, 1899. 
