886 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[May 10, 1873, 
soap or glycerine of starch of melting at the heat of the 
hand; that it is free from an organic body liable to de¬ 
compose like starch ; and that it does not irritate the 
skin like the alcoholic preparations. 
PROFESSOR TYNDALL ON LIGHT.* 
{Continued from p. 847.) 
We have employed as onr source of light in these 
lectures the ends of two rods of coke rendered mean- 
descent by electricity. Coke is particularly suitable 
for this purpose because it can bear intense heat without 
fusion or vaporization. It is also black, which helps the 
light; for other circumstances being equal, as shown by 
Balfour Stewart, the blacker the body the brighter will 
be its light when incandescent. Still refractory as carbon 
is, if we closely examined our voltaic arc, or stream of 
light between the carbon points, we should find there 
incandescent carbon vapour. We might also detach the 
light of this vapour from the more dazzling light of the 
solid points and obtain its spectrum. This would be not 
only less brilliant but of a character totally different from 
the spectra that we have already seen. Instead of being 
an unbroken succession of colours from red to violet, the 
carbon vapour would yield a few bands of colour with 
spaces of darkness between. 
What is true of the carbon is true in a still more 
striking degree of the metals, the most refractory of 
which can be fused, boiled, and reduced to vapour by the 
electric current. Prom the incandescent vapour the light, 
as a general rule, flashes in groups of rays of definite 
degrees of refrangibility, spaces existing between group 
and group which are unfilled by rays of any kind ; 
but the contemplation of the facts will render this subject 
more intelligible than words can make it. Within the 
camera is now placed a cylinder of carbon hollowed 
out at the top to receive a bit of metal ; in the hollow 
I put a fragment of the metal thallium, and now you 
see the arc of incandescent thallium vapour upon the 
screen. It is of a beautiful green colour. What is the 
meaning of that green ? We answer the question by sub¬ 
jecting the light to prismatic analysis ; here you have its 
spectrum, and it consists, as you see, of a single refracted 
band. Light of one degree of refrangibility, and that 
corresponding to green is emitted by the thallium vapour. 
We will now remove the thallium and put a bit of 
silver in its place. First observe the arc of silver ; it is 
not to be distinguished from that of thallium ; it is not 
only green, like the thallium vapour, but the same shade of 
green. Are they then alike ? Prismatic analysis enables 
us to answer the question. It is perfectly impossible to 
confound the spectrum of incandescent silver vapour 
with that of thallium. Here are two green bands in¬ 
stead of one. Adding to the silver in our camera a bit 
of thallium we obtain the light of both metals, and you 
see that the green of the thallium lies midway between 
the two greens of the silver. Hence this similarity of 
colour. But you observe another interesting fact. The 
thallium band is far brighter than the silver bands ; in¬ 
deed, the latter have wonderfully degenerated since the 
bit of thallium has been put in. They are not at all so 
bright as they were at first, and for a reason worth 
knowing. It is the resistance offered to the passage of 
the electric current from carbon to carbon that calls 
forth the power of the current to produce heat. If the 
resistance were materially lessened the heat would be 
materially lessened, and if all resistance were abolished, 
there would be no heat at all. Now thallium is a much 
more fusible and vaporizable a metal than the silver ; 
and its vapour facilitates the passage of the current to 
* Abstract of a series of lectures delivered in the Cooper 
Institute, New York, and reported in the New York 
Tribune. 
such a degree as to render it almost incompetent to 
vaporize silver. But the thallium is gradually consumed;, 
its vapour becomes less and less ; the resistance rises,, 
until finally you see the two silver bands as brilliant as 
they were at first. The three bands of the two metals are 
now of the same sensible brightness. 
We have in these bands a perfectly unalterable charac¬ 
teristic of these two metals. You never get other bands 
than these two green ones from the silver, never other 
than the single green band from the thallium, never other 
than the three green bands that you have just seen from 
the mixture of both metals. Every known metal has its 
bands, and in no known case are the bands of two 
different metals alike. Hence these spectra may be made 
a test as to the presence or absence of any particular 
metal. If we pass from the metals to their alloys we. 
find no confusion. Copper gives us green bands, zinc- 
gives us blue and red bands ; brass, an alloy of copper 
and zinc, gives us the bands of both metals, perfectly 
unaltered in position or character. But we are not con¬ 
fined to the metals; the salts of these metals yield the 
bands of the metals. Chemical union is ruptured by a 
sufficiently high heat, the vapour of the metal is set free 
and yields its characteristic bands. 
The chlorides of the metals are particularly suitable 
for experiments of this character. Common salt, for 
example, is a compound of chlorine and sodium ; in the- 
electric lamp it yields the spectrum of the metal sodium.. 
The chlorides of lithium and of strontium yield in like 
manner the bands of those metals. When, therefore, 
Bunsen and KirchhofF, after having established by am 
exhaustive examination the spectra of all known sub¬ 
stances, discovered a spectrum whose bands did not cor¬ 
respond to any known bands they immediately inferred 
the existence of a new metal. They were operating at 
the time upon a residue obtained by evaporating one ®f 
the mineral waters of Germany. In that water they 
knew the new metal was concealed, but vast quantities, 
of it had to be evaporated before a residue could be ob¬ 
tained sufficient to enable ordinary chemistry to grapple- 
with the metal. But they hunted it down, and it now- 
stands among chemical substances as the metal rubi¬ 
dium. They subsequently discovered a second metal, which 
they called caesium. Thus, having first placed spectrum, 
analysis on a safe foundation, they demonstrated its- 
capacity as an agent of discovery. Soon afterward Mr, 
Crookes, pursuing this same method, added to the list o£ 
metals the thallium which yielded that bright monochro¬ 
matic green band. 
This relates to chemical discovery upon the earth, where 
the materials are in our own hands. But Kirchhoff 
showed how spectrum analysis might be applied to the 
investigation of the sun and stars, and on his way to this, 
result he solved a problem which had been long an enigma 
to natural philosophers. A spectrum is pure in which 
the colours do not overlap each other. We purify the 
spectrum by making our slits narrow and by augmenting 
the number of our prisms. When a pure spectrum of the-, 
sun has been obtained in this way it is found furrowed 
by innumerable dark lines. Four of them were first seem 
by Dr. Wollaston, but they were afterward multiplied and 
measured by Fraunhofer with such masterly skill that, 
they are now universally known as Fraunhofer’s lines. To 
give an explanation of these lines was, as I have said, a- 
problem which long challenged the attention of philoso¬ 
phers. 
Now, Kirchhoff had made thoroughly clear to his mind 
the principles which linked together the emission of light 
and the absorption of light; he had proved their insepa¬ 
rability for each particular kind of light and heat. He 
had proved for every specific ray of the spectrum, the 
doctrine that the body emitting a ray absorbed with spe¬ 
cial energy a ray of the same refrangibility. Consider 
then the effect of knowledge such as you now possess; 
upon a mind prepared like that of Kirchhoff. We have 
seen the incandescent vapours of metals emitting definite 
