354 
NATURE 
| FEBRUARY 7, 1907 
(1) The fact that spectral lines are in general approxi- 
mately sharp. 
(2) The fact that spectral lines are never 
sharp, but always have a finite physical width. 
(3) The fact that certain spectral lines are arranged in 
series and bands. after the manner described so perfectly 
by Balmer’s equation and its generalised forms. 
(4) The fact that increase of pressure causes a shift of 
spectral lines toward the red, as discovered by Humphreys 
and Mohler. 
(5) The fact that a magnetic field will transform single 
lines into multiple polarised lines, as discovered by 
Zeeman. 
(6) We come now to a group of phenomena which are 
not easily described under a single caption. I refer to 
phenomena such as those observed by Plticker and Hittorf, 
when they found one and the same gas in one and the 
same tube yielding very different spectra according to the 
mode in which the electric discharge was applied to make 
the gas luminous. In the same category doubtless belongs 
the extinction of air lines by the insertion of self-induction 
into the discharge circuit. Here may belong also the fact 
studied by Lenard and others, that the region near the 
electrode of an are gives a spectrum different from the 
region near the centre of the arc; the fact also that the 
perfectly 
se-called. “‘ spark lines’’ are introduced into an are by 
reducing the current to small values, a fact first studied 
by Hartmann. 
Certainly in this same category belongs the fact that 
the spectrum of an are is modified when the arc is 
surrounded by an atmosphere different from ordinary air. 
Here also lie the profound differences between are and 
spark spectra of the same element. 
Notwithstanding the fact that ‘‘ multiple spectra’’ is a 
term which has hitherto been en:ployed to describe the 
Plucker tube variations, I propose that we generalise it 
and use it to describe this entire group of facts. Since the 
name is sO appropriate, let us call the sixth fundamental 
phenomenon that of ‘‘ multiple spectra.’ 
(7) Any competent atom must allow us to infer the re- 
lations which have been proved to exist between spectral 
phenomena and atomic weights. 
(8) The phenomena of line 
bands. 
(9) The fact that heat alone, at least within the range 
of our highest artificial temperatures, produces character- 
istic spectra in only a few rare instances. 
These, briefly, are the parts of the spectroscopic super- 
structure for which a foundation is sought. These are the 
various parts which it is hoped will, some day, be cemented 
reversals and absorption 
together, by a simple and general theory, into a harmonious 
structure. 
But there is a final criterion, even more fundamental 
than any of those which have been mentioned, that such 
a theory must satisfy, namely, this hypothetical radiant 
atom must not in its behaviour, except as a very last re- 
sort, contradict any of the established principles of physical 
science, be they mechanical, electrical, or chemical. 
The principle of the conservation of energy must be 
satisfied, even if it is necessary to assign an undreamed of 
amount of energy to each atom; in like manner Newton’s 
third law is to be satisfied, even if the electromagnetic 
ether is called upon to furnish the reaction. 
But even with this added criterion, the preceding list of 
nine phenomena is confessedly incomplete; the only object 
of such a catalogue is to include those typical fundamental 
facts which ought, apparently, to follow as immediate con- 
sequences from the structure of the radiating body, so soon 
“as that structure is correctly guessed. Thus Doppler’s 
principle is omitted on the ground of its being rather a 
kinematic law, governing periodic disturbances in any 
medium than a dynamical fact to be explained in terms 
of atomic structure and forces. 
Having established a set of criteria by which we may 
estimate the fitness of a radiant atom, it would be interest- 
ing, if I were competent, and if time permitted, to pass 
in review some of the various atoms which have been pro- 
posed in recent times, such as that of Kelvin, 
those suggested by the Hertzian oscillator. 
But neither of these two conditions are fulfilled, and I 
propose, therefore, to consider only one atom, namely, the 
NO. 1945. VOL. 75] 
1884, or | 
| 
one which by common consent, I think I may safely say, 
more nearly satisfies the demands of experimental fact 
than any other ever devised. I refer to the atom first 
proposed in a general way by Lord Kelvin in his paper 
entitled ‘‘ Epinus Atomised ’’ (Baltimore Lectures, p. 541, 
Cambridge, 1904), and afterwards profoundly modified by 
Lorentz, Thomson, and Larmor. 
So much work along this line has been done in the 
Cavendish Laboratory that one feels impelled to call this 
“the Cambridge atom ’’; in view, however, of its struc- 
ture, perhaps ‘‘ the Saturnian atom ’’ is a more appropriate 
designation. 
AmeriIcA to GEOLOoGy.' 
In speaking of the contributions of America to geology, 
I do not propose to give an inventory of the geological 
facts which have been made known as the result of work 
in this country. I propose rather to ask the question, 
““What has our country contributed to the stock of geo- 
logical ideas?’’ In that classical history of geological 
science which Lyell has given us in his ‘‘ Principles of 
Geology,’’ he directs attention to the fact that the share 
which different nations bore in the early development of 
geological science was dependent, not alone upon the genius 
of individual workers, but in large measure upon the 
peculiar geological conditions of the various countries in 
which they worked. 
Of course, it must be admitted that there is to-day no 
department of geological science which is as character- 
istically American as mineralogy was German, as dynamical 
geology was Scotch, as stratigraphical geology was 
English, and as paleontology was French, a century ago. 
I believe, nevertheless, that there have been certain con- 
tributions to the stock of geological ideas which are 
characteristically American. 
The doctrine of the permanence of continent and ocean 
—the gradual emergence of continental lands and the with- 
drawal of the waters into the deepening ocean basins—was 
first enunciated by Dana in 1846. It was, apparently, the 
thought of the subsiding ocean bottom rather than the 
thought of the emerging land by which Dana was first 
led to the doctrine of the permanence of continent and 
ocean, but in his presidential address before the American 
Association for the Advancement of Science in 1855, Dana 
refers to the stratigraphy of New York as illustrating the 
idea of continental emergence. ‘The doctrine of the perman- 
ence of continents when announced by Dana was essentially 
a new one. Geologists and pseudo-geologists of all classes 
had felt at liberty to re-distribute continents and oceans 
according to their own sweet will. 
There is now little doubt that Dana was right in his general 
conception. The greater density of the suboceanic masses 
in comparison with the subcontinental masses, as shown 
by pendulum observations, indicates that the distinction 
between continent and ocean has its basis in the hetero- 
geneity of the material in the interior of the earth, and 
the determining conditions must therefore have had their 
origin in the initial aggregation of that part of the primi- 
tive nebula which formed the earth. 
Certain it is, however, that Dana made the evolution 
of the continents too simple an affair. He recognised, 
indeed, that the progressive emergence of the continental 
lands was attended by continual oscillation, yet, even in 
the last edition of his ‘‘ Manual,’’ it appears that he did 
not duly appreciate the magnitude of those oscillations. 
The doctrine of the progressive evolution of continents, 
as taught by Dana, gave new clearness and emphasis to 
the general conception of geology as a history of the 
globe. 
The Geological Survey of Pennsylvania made known the 
folded, structure—the alternate anticlines and synclines—of 
the Appalachians. The beautiful sections of these folded 
strata, in the atlas of that survey, reveal the thoroughness 
with which the structure of the mountains was investigated 
by Henry D. Rogers. 
While the stratigraphy was worked out so beautifully in 
the first geological survey of Pennsylvania, the dynamic 
conception derived from it was crude indeed. But, how- 
ever completely the Pennsylvania geologists failed to con- 
1 From an address delivered by Prof. Wm. North Rice, chairman of the 
Se-tion of Geology and Geography. 
Tue CONTRIBUTIONS OF 
