SCIENCE. 
219 
posed to the law of continuity, as I endeavored to show in 
the last paragraphs quoted, but it appears never to have 
struck the objectors that it is also opposed to the theory of 
exchanges as it is generally enunciated, on which the whole 
of our supposed knowledge of extra-terrestrial matter de- 
pends. If vapors, when relatively cool, do not absorb the 
same wave-lengths which they give out when relatively hot, 
what becomes of some of the most noted exploits of our 
nineteenth-century science ? 
Take the case of sodium. Three distinct spectra have 
been mapped for it. There is first the yellow line seen in 
a Bunsen flame, then the green line seen alone in a vacuum 
tube when the vapor is illuminated by an electric glow, and 
again there is the fluted absorption spectrum, without any 
lines, seen when sodium is gently heated in hydrogen in a 
glass tube. If we have here the same molecule agitated in 
different ways, I ask which is the true spectrum of sodium? 
And what right have we to say that sodium exists in the 
sun because the yellow line is represented ? Why do we 
not rather say that sodium does not exist in the sun because 
the fluted spectrum is not represented. 
a b c d e f g h k l m 
Fig. 1 . — A. Highest temperature. C. Lowest temperature. 
It is not necessary to enlarge upon this point because 
the difficulty in which the theory of exchanges is thus landed 
is obvious, while, if we acknowledge different molecular 
groupings in the vapors of the same chemical substance, 
and apply the theory of exchanges to each grouping, then the 
teachings of that theory become more numerous and im- 
portant than before. 
It is of course of the highest importance to see whether 
there is any experimentum crucis — any mode of inquiry — by 
which the theory can be settled one way or the other. 
I submit that the results of experiments based on the fol- 
lowing considerations ought to be accepted as throwing 
light on the question. 
1. At different temperatures the brilliancy of the spectral 
lines of the same substances as ordinarily observed changes 
enormously. See if these changes can be produced at the 
same temperature by employing those experimental condi- 
tions which will be most likely to bring about different 
molecular conditions if such exist. 
2. At a low temperature some substances give us few 
lines while at a high one they give us many. Vapors, 
therefore, already glowing with full lines at a low tempera- 
ture, say in a flame, should give us all their lines when the 
vapor is suddenly subjected to a high one, say by the 
passage of a high tension spark. On the bell hypothesis 
the spectrum should change with the mode of striking. On 
the dissociation hypothesis this should only happen for the 
lines of those molecular groupings which are from other 
considerations held to be more simple. If the flame has 
brought the substance to its lowest state, the passage of the 
most powerful spark should not cause the flame spectrum 
to vary. 
Now what are the “other considerations” above referred 
to ? This necessitates a slight digression. 
In the Phil. Trans, for 1873 1 I gave an historical account, 
showing how, when a light source such as a spark or an 
electric arc is made to throw its image on the slit of a spec- 
troscope, the lines had been seen of different lengths, and I 
also showed by means of photographs how very definite 
these phenomena were. It was afterwards demonstrated 
that for equal temperatures chemical combination or 
mechanical mixture gradually reduced the spectrum by sub- 
tracting the shortest lines, and leaving only the long ones. 
On the hypothesis that the elements were truly elemen- 
tary, the explanation generally given and accepted was that 
the short lines were produced by a more complex vibration 
imparted to the “atom” in the region of greatest electrical 
excitement, and that these vibrations were obliterated, or 
prevented from arising, by cooling or admixture with dis- 
similar “ atoms.” 
Subsequent work, however, has shown 1 that of these 
short lines some are common to two or more spectra. These 
lines I have called basic. Among the short lines, then, we 
have some which are basic, and some which are not. 
The different behavior of these basic lines seemed, there- 
fore, to suggest that not all of the short lines of spectra were, 
in reality, true products of high temperature. 
That some would be thus produced and would therefore 
be common to two or more spectra we could understand 
by appealing to Newton’s rule: “ Causas rerum natu- 
ralium non plures admitti debere quam quae et varae sint et 
earum phaenomenis explicandis sufficiant,” and imagining 
a higher dissociation. It became, however, necessary to 
see if the others would also be accounted for. 
Now if not all but only some of the short lines are pro- 
ducts of high temperature, we are bound to think that the 
others are remnants of the spectra of those molecular group- 
ings first to disappear on the application of heat. 
At any particular heat-level, then, some of the short lines 
may be due to the vibrations of molecular groupings pro- 
duced with difficulty by the temperature employed, while 
others may represent the fading out of the vibrations of 
other molecular groupings, produced on the first applica- 
tion of the heat. 
In the line of reasoning which I advanced a year ago, 2 
both these results are anticipated, and are easily explained. 
Slightly var)dng Fig. 2 of that paper, we may imagine furn- 
ace A to represent the temperature of the jar spark, B that 
of the Bunsen burner, and C a temperature lower than that 
of the Bunsen burner (Fig. 1.) 
Then in the light of the paper the lines b and c would be 
truly produced by the action of the highest temperature, c 
would be short and might be basic, while of the lines h and 
m, in would be short and could not be basic, because it is a 
remnant of the spectrum of a lower temperature. 
So much then bjr way of explanation ; it is clear that to 
make this reasoning valid we must show that the spark, or 
better still the arc, provides us with as ummation of the spec- 
tra of various molecular groupings into which the solid 
metal which we use as poles is successively broken up by the 
action of heat. 
We are not limited to solid metals ; we may use their 
salts. In this case it is shown in the paper before referred 
to 3 that in very many cases the spectrum is one much less 
rich in lines. 
The experimental work has followed two distinct lines. 
I shall refer somewhat in detail to the results obtained along 
each. The first relates to the extraordinary and beautiful 
phenomena and changes observed in the spectra of vapors 
of the elementary bodies when volatilized at different tem- 
peratures in vacuum tubes. Many of the lines thus seen 
alone and of surpassing brilliancy, are those seen as short 
and faint in ordinary methods of observation, and the cir- 
cumstances under which they are seen suggest, if we again 
apply Newton’s rule, that many of them are produced by 
complex molecules. 
In this case the appeal lies to the phenomena produced 
when organic bodies are distilled at varying temperatures ; 
the simplest bodies in homologous series are those volatil- 
ized at the lowest temperatures ; so that on subjecting a 
mixture of two ormore liquids to distillation, at the begin- 
ning a large proportion of the more volatile body comes 
over, and so on. 
The novelty of the method consists in the use of the 
luminous electric current as an explorer and not as an 
agent for the supply of the vapors under examination ; 
that is to say, the vapors are first produced by an external 
source of heat, and are then rendered luminous by the 
passage of the current. The length and bore of the tube 
therefore control the phenomena to a certain extent. 
1 Proc. R.S.,v ol. xxviii. p. 159. 2 Proc. R. S., vol. xxviii. p, 162. 
3 Phil. Trans., 1873, p. 258. 
1 Phil. Trans., 1873, p. 254. 
