to what a theory is or ought to be. I call a theory a principle or
conception of the mind which accounts for observed facts, and which
helps us to look for and predict facts not yet observed. Every new
discovery which fits into a theory strengthens it. The theory is not
a thing complete from the first, but a thing which grows, as it were
asymptotically, towards certainty. Darwin's theory, as pointed out
nine and ten years ago by Helmholtz and Hooker, was then exactly in
this condition of growth; and had they to speak of the subject to-day
they would be able to announce an enormous strengthening of the
theoretic fibre. Fissures in continuity which then existed, and which
left little hope of being ever spanned, have been since filled in, so
that the further the theory is tested the more fully does it harmonise
with progressive experience and discovery. We shall probably never
fill all the gaps; but this will not prevent a profound belief in the
truth of the theory from taking root in the general mind. Much less
will it justify a total denial of the theory. The man of science who
assumes in such a case the position of a denier is sure to be stranded
and isolated. The proper attitude, in my opinion, is to give to the
theory during the phases of its growth as nearly as possible a
proportionate assent; and, if it be a theory which influences
practice, our wisdom is to follow its probable suggestions where more
than probability is for the moment unattainable. I write thus with
the theory of _contagium vivum_, more especially in my mind, and must
regret the attitude of denial assumed by Professor Virchow towards
that theory. 'I must beg my friend Klebs to pardon me,' he says, 'if,
notwithstanding the late advances made by the doctrine of infectious
fungi, I still persist in my reserve so far as to admit only the
fungus which is really proved while I deny all other fungi so long as
they are not actually brought before me.' Professor Virchow, that is
to say, will continue to deny the Germ Theory, however great the
probabilities on its side, however numerous be the cases of which it
renders a just account, until it has ceased to be a theory at all, and
has become a congeries of sensible facts. Had he said, 'As long as a
single fungus of disease remains to be discovered, it is your bounden
duty to search for it,' I should cordially agree with him. But by his
unreserved denial he quenches the light of probability which ought to
guide the practice of the medical man. Both here and in relation to
the theory of evolution excess upon one side has begotten excess upon
the other.
====================
NOTE.--As might have been expected, Professor Virchow, shows himself
in practice far too sound a philosopher to be restricted by the canon
laid down in his critique of Dr. Haeckel. In his recent discourse
upon the plague, he asks and answers the question, 'What is the
_contagium_?' in the following words: 'Et qu'est-ce que le _contagium_? A
mon avis, l'analogie de la peste aver le charbon contagieux me paraît
si grande qu'il me semble possible de trouver un organisme
microscopique qui contient le germe de l'affection. Mais jusqu' à
présent on a peu cherché à trouver cet organisme.'--Revue
Scientifique, March, 1879.
********************
XVI. THE ELECTRIC LIGHT.
[Footnote: A discourse delivered at the Royal Institution of Great
Britain on Friday, January 17, 1879, and introduced here as the latest
Fragment.]
THE subject of this evening's discourse was proposed by our late
honorary secretary. [Footnote: Mr. William Spottiswoode, now President
of the Royal Society] That word 'late' has for me its own
connotations. It implies, among other things, the loss of a comrade
by whose side I have worked for thirteen years. On the other hand,
regret is not without its opposite in the feeling with which I have
seen him rise by sheer intrinsic merit, moral and intellectual, to the
highest official position which it is in the power of English science
to bestow. Well, he, whose constant desire and practice were to
promote the interests and extend the usefulness of this institution,
thought that at a time when the electric light occupied so much of
public attention, a few sound notions regarding it, on the more purely
scientific side, might, to use his own pithy expression, be 'planted'
in the public mind. I am here to-night with the view of trying, to
the best of my ability, to realise the idea of our friend.
In the year 1800, Volta announced his immortal discovery of the pile.
Whetted to eagerness by the previous conflict between him and Galvani,
the scientific men of the age flung themselves with ardour upon the
new discovery, repeating Volta's experiments, and extending them in
many ways. The light and heat of the voltaic circuit attracted marked
attention, and in the innumerable tests and trials to which this
question was subjected, the utility of platinum and charcoal as means
of exalting the light was on all hands recognised. Mr. Children, with
a battery surpassing in strength all its predecessors, fused platinum
wires eighteen inches long, while 'points of charcoal produced a light
so vivid that the sunshine, compared with it, appeared feeble.'
[Footnote: Davy, 'Chemical Philosophy,' p. 110.] Such effects
reached their culmination when, in 1808, through the liberality of a
few members of the Royal Institution, Davy was enabled to construct a
battery of two thousand pairs of plates, with which he afterwards
obtained calorific and luminous effects far transcending anything
previously observed. The arc of flame between the carbon terminals
was four inches long, and by its heat quartz, sapphire, magnesia, and
lime, were melted like wax in a candle flame; while fragments of
diamond and plumbago rapidly disappeared as if reduced to vapour.
[Footnote: In the concluding lecture at the Royal Institution in June,
1810, Davy showed the action of this battery. He then fused iridium,
the alloy of iridium and osmium, and other refractory substances.
'Philosophical Magazine,' vol. xxxv. p. 463. Quetelet assigns the
first production of the spark between coal-points to Curtet in 1802.
Davy certainly in that year showed the carbon light with a battery of
150 pairs of plates in the theatre of the Royal Institution ('Jour.
Roy. Inst.' vol. i. p. 166).]
The first condition to be fulfilled in the development of heat and
light by the electric current is that it shall encounter and overcome
resistance. Flowing through a perfect conductor, no matter what the
strength of the current might be, neither heat nor light could be
developed. A rod of unresisting copper carries away uninjured and
unwarmed an atmospheric discharge competent to shiver to splinters a
resisting oak. I send the self-same current through a wire composed
of alternate lengths of silver and platinum. The silver offers little
resistance, the platinum offers much. The consequence is that the
platinum is raised to a white heat, while the silver is not visibly
warmed. The same holds good with regard to the carbon terminals
employed for the production of the electric light. The interval
between them offers a powerful resistance to the passage of the
current, and it is by the gathering up of the force necessary to burst
across this interval that the voltaic current is able to throw the
carbon into that state of violent intestine commotion which we call
heat, and to which its effulgence is due. The smallest interval of
air usually suffices to stop the current. But when the carbon points
are first brought together and then separated, there occurs between
them a discharge of incandescent matter which carries, or may carry,
the current over a considerable space. The light comes almost wholly
from the incandescent carbons. The space between them is filled with
a blue flame which, being usually bent by the earth's magnetism,
receives the name of the Voltaic Arc. [Footnote: The part played by
resistance is strikingly illustrated by the deportment of silver and
thallium when mixed together and volatilised in the arc. The current
first selects as its carrier the most volatile metal, which in this
case is thallium. While it continues abundant, the passage of the
current is so free--the resistance to it is so small--that the heat
generated is incompetent to volatilise the silver. As the thallium
disappears the current is forced to concentrate its power; it presses
the silver into its service, and finally fills the space between the
carbons with a vapour which, as long as the necessary resistance is
absent, it is incompetent to produce. I have on a former occasion
drawn attention to a danger which besets the spectroscopist when
operating upon a mixture of constituents volatile in different
degrees. When, in 1872, I first observed the effect here described,
had I not known that silver was present, I should have inferred its
absence.]
For seventy years, then, we have been in possession of this
transcendent light without applying it to the illumination of our
streets and houses. Such applications suggested themselves at the
outset, but there were grave difficulties in their way. The first
difficulty arose from the waste of the carbons, which are dissipated
in part by ordinary combustion, and in part by the electric transfer
of matter from the one carbon to the other. To keep the carbons at
the proper distance asunder regulators were devised, the earliest, I
believe, by Staite, and the most successful by Duboscq, Foucault, and
Serrin, who have been succeeded by Holmes, Siemens, Browning, Carré,
Gramme, Lontin, and others. By such arrangements the first difficulty
was practically overcome; but the second, a graver one, is probably
inseparable from the construction of the voltaic battery. It arises
from the operation of that inexorable law which throughout the
material universe demands an eye for an eye, and a tooth for a tooth,
refusing to yield the faintest glow of heat or glimmer of light
without the expenditure of an absolutely equal quantity of some other
power. Hence, in practice, the desirability of any transformation
must depend upon the value of the product in relation to that of the
power expended. The metal zinc can be burnt like paper; it might be
ignited in a flame, but it is possible to avoid the introduction of
all foreign heat and to burn the zinc in air of the temperature of
this room. This is done by placing zinc foil at the focus of a
concave mirror, which concentrates to a point the divergent electric
beam, but which does not warm the air. The zinc burns at the focus
with a violet flame, and we could readily determine the amount of heat
generated by its combustion. But zinc can be burnt not only in air
but in liquids. It is thus burnt when acidulated water is poured over
it; it is also thus burnt in the voltaic battery. Here, however, to
obtain the oxygen necessary for its combustion, the zinc has to
dislodge the hydrogen with which the oxygen is combined. The
consequence is that the heat due to the combustion of the metal in the
liquid falls short of that developed by its combustion in air, by the
exact quantity necessary to separate the oxygen from the hydrogen.
Fully four-fifths of the total heat are used up in this molecular
work, only one-fifth remaining to warm the battery. It is upon this
residue that we must now fix our attention, for it is solely out of it
that we manufacture our electric light.
Before you are two small voltaic batteries of ten cells each. The two
ends of one of them are united by a thick copper wire, while into the
circuit of the other a thin platinum wire is introduced. The platinum
glows with a white heat, while the copper wire is not sensibly warmed.
Now an ounce of zinc, like an ounce of coal, produces by its complete
combustion in air a constant quantity of heat. The total heat
developed by an ounce of zinc through its union with oxygen in the
battery is also absolutely invariable. Let our two batteries, then,
continue in action until an ounce of zinc in each of them is consumed.
In the one case the heat generated is purely domestic, being liberated
on the hearth where the fuel is burnt, that is to say in the cells of
the battery itself. In the other case, the heat is in part domestic
and in part foreign--in part within the battery and in part outside.
One of the fundamental truths to be borne in mind is that the sum of
the foreign and domestic--of the external and internal--heats is fixed
and invariable. Hence, to have heat outside, you must draw upon the
heat within. These remarks apply to the electric light. By the
inter-mediation of the electric current the moderate warmth of the
battery is not only carried away, but concentrated, so as to produce,
at any distance from its origin, a heat next in order to that of the
sun. The current might therefore be defined as the swift carrier of
heat. Loading itself here with invisible power, by a process of
transmutation which outstrips the dreams of the alchemist, it can
discharge its load, in the fraction of a second, as light and heat, at
the opposite side of the world.
Thus, the light and heat produced outside the battery are derived from
the metallic fuel burnt within the battery; and, as zinc happens to be
an expensive fuel, though we have possessed the electric light for
more than seventy years, it has been too costly to come into general
use. But within these walls, in the autumn of 1831, Faraday
discovered a new source of electricity, which we have now to
investigate. On the table before me lies a coil of covered copper
wire, with its ends disunited. I lift one side of the coil from the
table, and in doing so exert the muscular effort necessary to overcome
the simple weight of the coil. I unite its two ends and repeat the
experiment. The effort now required, if accurately measured, would be
found greater than before. In lifting the coil I cut the lines of the
earth's magnetic force, such cutting, as proved by Faraday, being
always accompanied, in a closed conductor, by the production of an
'induced' electric current which, as long as the ends of the coil
remained separate, had no circuit through which it could pass. The
current here evoked subsides immediately as heat; this heat being the
exact equivalent of the excess of effort just referred to as over and
above that necessary to overcome the simple weight of the coil. When
the coil is liberated it falls back to the table, and when its ends
are united it encounters a resistance over and above that of the air.
It generates an electric current opposed in direction to the first,
and reaches the table with a diminished shock. The amount of the
diminution is accurately represented by the warmth which the momentary
current developer in the coil. Various devices were employed to exalt
these induced currents, among which the instruments of Pixii, Clarke,
and Saxton were long conspicuous. Faraday, indeed, foresaw that such
attempts were sure to be made; but he chose to leave them in the hands
of the mechanician, while he himself pursued the deeper study of facts
and principles. 'I have rather,' he writes in 1831, 'been desirous
of discovering new facts and new relations dependent on
magneto-electric induction, than of exalting the force of those
already obtained; being assured that the latter would find their full
development hereafter.'
For more than twenty years magneto-electricity had subserved its first
and noblest purpose of augmenting our knowledge of the powers of
nature. It had been discovered and applied to intellectual ends, its
application to practical ends being still unrealised. The Drummond
light had raised thoughts and hopes of vast improvements in public
illumination. Many inventors tried to obtain it cheaply; and in 1853
an attempt was made to organise a company in Paris for the purpose of
procuring, through the decomposition of water by a powerful
magneto-electric machine constructed by M. Nollet, the oxygen and
hydrogen necessary for the lime light. The experiment failed, but the
apparatus by which it was attempted suggested to Mr. Holmes other and
more hopeful applications. Abandoning the attempt to produce the lime
light, with persevering skill Holmes continued to improve the
apparatus and to augment its power, until it was finally able to yield
a magneto-electric light comparable to that of the voltaic battery.
Judged by later knowledge, this first machine would be considered
cumbrous and defective in the extreme; but judged by the light of
antecedent events, it marked a great step forward.
Faraday was profoundly interested in the growth of his own discovery.
The Elder Brethren of the Trinity House had had the wisdom to make him
their 'Scientific Adviser;' and it is interesting to notice in his
reports regarding the light, the mixture of enthusiasm and caution
which characterised him. Enthusiasm was with him a motive power,
guided and controlled by a disciplined judgment. He rode it as a
charger, holding it in by a strong rein. While dealing with Holmes,
he states the case of the light pro and con. He checks the ardour of
the inventor, and, as regards cost, rejecting sanguine estimates, he
insists over and over again on the necessity of continued experiment
for the solution of this important question. His matured opinion was,
however, strongly in favour of the light. With reference to an
experiment made at the South Foreland on the 20th of April, 1859, he
thus expresses himself: 'The beauty of the light was wonderful. At
a mile off, the Apparent streams of light issuing from the lantern
were twice as long as those from the lower lighthouse, and apparently
three or four times as bright. The horizontal plane in which they
chiefly took their way made all above or below it black. The tops of
the bills, the churches, and the houses illuminated by it were
striking in their effect upon the eye.' Further on in his report he
expresses himself thus: 'In fulfilment of this part of my duty, I beg
to state that, in my opinion, Professor Holmes has practically
established the fitness and sufficiency of the magneto-electric light
for lighthouse purposes, so far as its nature and management are
concerned. The light produced is powerful beyond any other that I
have yet seen so applied, and in principle may be accumulated to any
degree; its regularity in the lantern is great; its management easy,
and its care there may be confided to attentive keepers of the
ordinary degree of intellect and knowledge.' Finally, as regards the
conduct of Professor Holmes during these memorable experiments, it is
only fair to add the following remark with which Faraday closes the
report submitted to the Elder Brethren of the Trinity House on the
29th of April, 1859: 'I must bear my testimony,' he says, 'to the
perfect openness, candour, and honour of Professor Holmes. He has
answered every question, concealed no weak point, explained every
applied principle, given every reason for a change either in this or
that direction, during several periods of close questioning, in a
manner that was very agreeable to me, whose duty it was to search for
real faults or possible objections, in respect both of the present
time and the future.' [Footnote: Holmes's first offer of his machine
to the Trinity House bears date February 2, 1857.]
Soon afterwards the Elder Brethren of the Trinity House had the
intelligent courage to establish the machines of Holmes permanently at
Dungeness, where the magneto-electric light continued to shine for
many years.
The magneto-electric machine of the Alliance Company soon succeeded to
that of Holmes, being in various ways a very marked improvement on the
latter. Its currents were stronger and its light was brighter than
those of its predecessor. In it, moreover, the commutator, the
flashing and destruction of which were sources of irregularity and
deterioration in the machine of Holmes, was, at the suggestion of M.
Masson, entirely abandoned; alternating currents instead of the direct
current being employed. [Footnote: Du Moncel, 'l'Electricité,'
August, 1878, p. 150.] M. Serrin modified his excellent lamp with
the express view of enabling it to cope with alternating currents.
During the International Exhibition of 1862, where the machine was
shown, M. Berlioz offered to dispose of the invention to the Elder
Brethren of the Trinity House. They referred the matter to Faraday,
and he replied as follows: 'I am not aware that the Trinity House
authorities have advanced so far as to be able to decide whether they
will require more magneto-electric machines, or whether, if they
should require them, they see reason to suppose the means of their
supply in this country, from the source already open to them, would
not be sufficient. Therefore I do not see that at present they want
to purchase a machine.' Faraday was obviously swayed by the desire to
protect the interests of Holmes, who had borne the burden and heat
which fall upon the pioneer. The Alliance machines were introduced
with success at Cape la Hève, near Havre; and the Elder Brethren of
the Trinity House, determined to have the best available apparatus,
decided, in 1868, on the introduction of machines on the Alliance
principle into the lighthouses at Souter Point and the South Foreland.
These, machines were constructed by Professor Holmes, and they still
continue in operation. With regard, then, to the application of
electricity to lighthouse purposes, the course of events was this: The
Dungeness light was introduced on January 31, 1862; the light at La
Hève on December 26, 1863, or nearly two years later. But Faraday's
experimental trial at the South Foreland preceded the lighting of
Dungeness by more than two years. The electric light was afterwards
established at Cape Grisnez. The light was started at Souter Point on
January 11, 1871; and at the South Foreland on January 1, 1872.
At the Lizard, which enjoys the newest and most powerful development
of the electric light, it began to shine on January 1, 1878.
*****