THE RELATION BETWEEN RADIANT HEAT
AND ELECTRIC WAVES
In this concluding chapter it is proposed to show how the wave-lengths
of radiant heat have been determined and to state what range of
wave-lengths has been experimentally observed. It is then proposed to
show how electromagnetic waves have been produced by straightforward
electrical means and how their wave-lengths have been measured. The
similarity in properties of the radiant heat and of the electric waves
will be noted, leading to the conclusion that the difference between
the two sets of waves is merely one of wave-length.
+Diffraction Grating.+--The best method of measuring the wave-lengths
of heat and light is by means of the "Diffraction Grating." This
consists essentially of a large number of fine parallel equidistant
slits placed very close to one another. For the measurement of the
wave-lengths of light and of the shorter heat waves, it is usually
produced by ruling a large number of very fine close equidistant lines
on a piece of glass or on a polished mirror by means of a diamond
point. The ruled lines are opaque on the glass and do not reflect on
the mirror, and consequently the spaces in between act as slits.
{73}
+Rowland's Gratings.+--The ruling of these gratings is a very difficult
and tedious business, but the difficulties have been surmounted in a
very remarkable manner by Rowland, so that the gratings ruled on his
machine have become standard instruments throughout the world. He
succeeded in ruling gratings 6 inches in diameter with 14,000 lines to
the inch, truly a remarkable performance when we remember that if the
diamond point develops the slightest chip in the process, the whole
grating is spoilt.
[Illustration: FIG. 25.]
The action of the grating can be made clear by means of Fig. 25. Let
A, B, C, D represent the {74} equidistant slits in a grating, and let
the straight lines to the left of the grating represent at any instant
the crests of some simple plane waves coming up to the grating. The
small fractions of the original waves emerging from the slits A, B, C,
D will spread out from the slits so that the crests of the small
wavelets may at any instant be represented by a series of concentric
circles, starting from each slit as centre. The series of crests from
each slit are represented in the figure.
Now notice that a line PQ parallel to the original waves lies on one of
the crests from each slit, and therefore the wavelets will make up a
plane wave parallel to the original wave. This may therefore be
brought to a focus by means of a convex lens just as if the grating
were removed, except that the intensity of the wave is less. But a
line, LM, also lies on a series of crests, the crest from A being one
wave-length behind that from B, the one from B a wave-length behind
that from C, and so on. The wavelets will therefore form a plane wave
LM, which will move in the direction perpendicular to itself (_i.e._
the direction DK) and may be brought to a focus in that direction by
means of a lens.
Draw CH and DK perpendicular to LM, and draw CE perpendicular to DK,
_i.e._ parallel to LM. The difference between CH and DK is evidently
one wave-length, _i.e._ DE is one wave-length. If [Greek: alpha] is
the angle between the direction of PQ and LM, DE is evidently equal to
CD sin [Greek: alpha] and therefore one wave-length=CD sin [Greek:
alpha].
From the ruling of the grating we know the value {75} of CD, and
therefore by measuring [Greek: alpha] we can calculate the wave-length.
We find that a third line RS also lies on a series of crests, and
therefore a plane wave sets out in the direction perpendicular to RS.
We notice here that the crest from A is two wave-lengths behind that
from B, and so on, and therefore if [Greek: beta] is the angle between
RS and PQ, CD sin [Greek: beta] is equal to two wave-lengths.
Similarly we get another plane wave for a three wave-lengths
difference, and so on. The intensity of the wavelets falls off fairly
rapidly as they become more oblique to their original direction, and
therefore the intensity of these plane waves also falls off rather
rapidly as they become more oblique to the direction in which PQ goes.
We see that the essential condition for the plane wave to set out in
any direction, is that the difference in the distances of the plane
wave from two successive slits shall be exactly a whole number of
wave-lengths. Should it depart ever so little from this condition we
should see, on drawing the line, that there lie on the line an equal
number of crests and troughs, and therefore, if a lens focus waves in
this direction, the resulting effect is zero. The directions of the
waves PQ, LM, RS, &c., will therefore be very sharply defined and will
admit of very accurate determination.
+Dispersion by Grating.+--Evidently the deviations [Greek: alpha],
[Greek: beta] will be greater the greater is DE, _i.e._ the greater the
wave-length, and therefore the light or heat will be "dispersed" into
its different wave-lengths as in the prism; but in this case the
dispersion {76} is opposite to that in the normal prism, the long waves
being dispersed most and the short waves least.
Evidently, too, the smaller the distance CD the greater the angle, and
therefore for the extremely short wave-lengths of light and of
ultraviolet rays we require the distance between successive slits to be
extremely small.
[Illustration: FIG. 26.]
+The Spectrometer.+--The grating is usually used with a spectrometer,
as shown in plan diagrammatically in Fig. 26. The slit S from which
the waves radiate is placed at the principal focus of the lens L, and
therefore the waves emerge from L as plane waves which come up to the
grating G. The telescope T is first turned until it views the slit
directly, _i.e._ until the plane waves like PQ in Fig. 25 are brought
to a focus at the principal focus F of the objective of the telescope.
The eyepiece E views the image of the slit S which is formed at F. The
telescope is then turned through an angle, [Greek: alpha], until it
views the second image of the slit which will be formed by the plane
waves similar to LM in Fig. 25. The angle [Greek: alpha] is carefully
measured by the graduated circle on the spectrometer, {77} and hence
the wave-length of a particular kind of light, or of a particular part
of the spectrum, is measured.
This spectrometer method is exactly the method used for measuring the
wave-lengths in the visible part of the spectrum.
For the ultraviolet rays, instead of viewing the image of the slit by
means of the eyepiece of the telescope, a photographic plate is placed
at the principal focus F of the objective of the telescope, and serves
to detect the existence and position of these shorter waves. For the
heat rays a Langley's bolometer strip is placed at F, in fact the
bolometer strip might be used throughout, but it is not quite so
sensitive for the visible and ultraviolet rays as the eye and the
photographic plate.
+Absorption by Glass and Quartz.+--Two main difficulties arise in these
experiments. The first one is that although glass, or better still
quartz, is extremely transparent to ultraviolet, visible, and the
shorter infra-red waves, yet it absorbs some of the longer heat waves
almost completely.
For these waves, therefore, some arrangement must be devised in which
they are not transmitted through a glass diffraction grating or through
glass or quartz lenses. To effect this, the convex lenses are replaced
by concave mirrors and the ruled grating is replaced by one which is
made of very fine wires, which are stretched on a frame parallel to and
equidistant from each other. The wire grating cannot be constructed
with such fine or close slits as the ruled grating, but for the longer
waves this is unnecessary.
{78}
+Reflecting Spectrometer.+--An arrangement used by Rubens is
represented roughly in plan in Fig. 27. L represents the source of
heat, the rays from which are reflected at the concave mirror M, and
brought to a focus on the slit S. Emerging from S the rays are
reflected at M(sub)2 and are thereby rendered parallel before passing
through the wire grating G. After passing through the grating, the
rays are reflected at M(sub)3 and are thereby focussed on to a
bolometer strip placed at B. Turning the mirror M(sub)3 in this
arrangement is evidently equivalent to turning the telescope in the
ordinary spectrometer arrangement.
[Illustration: FIG. 27.]
+Absorption of Waves by Air.+--By using a spectrometer in an exhausted
vessel Schumann discovered that waves existed in the ultraviolet region
of much smaller wave-length than any previously found, and that these
waves were almost completely absorbed on passing through a few
centimetres of air. To all longer waves, however, air seems to be
extremely transparent.
The second difficulty arises from the fact, already explained, that a
diffraction grating produces not one, but a number of spectra. If only
a small range of waves exists, this will lead to no confusion, but if a
large range is being investigated, we may get two or more of these
spectra overlapping.
Suppose, for example, we have some waves of wave-length DE (in Fig.
25), some of wave-length one-half {79} DE and some of one-third DE.
Then in the direction DK we shall get plane waves of each of these
wave-lengths setting out and being brought to a focus in the same
place. This difficulty can be fairly simply surmounted where the
measurement of wave-length alone is required, by placing in the path of
the rays from the source of light, suitable absorbing screens, which
will only allow a very small range of wave-lengths to pass through
them. There will then be no overlapping and no confusion.
Where the actual distribution of energy in the spectrum of any source
of heat is to be determined the difficulty becomes more serious, and
probably there is some error in the determinations, especially in the
longest waves, which are masked almost completely by the overlapping
shorter waves.
+Rest-Strahlen or Residual Rays.+--A very beautiful method of isolating
very long heat waves, and so freeing them from the masking effect of
the shorter waves, was devised by Rubens and Nichols.
It is found that when a substance very strongly absorbs any waves that
pass through it, it also strongly reflects at its surface the same
waves. For example, a sheet of glass used as a fire-screen will cut
off most of the heat coming from the fire, although it is perfectly
transparent to the light. If, now, it is placed so as to reflect the
light and heat from the fire, it is found to reflect very little light
but a very large proportion of the heat.
Some substances have a well-defined absorption band, _i.e._ they absorb
a particular wave-length very strongly, and these substances will
therefore reflect {80} this same wave-length strongly. If instead of a
single reflection a number of successive reflections be arranged, at
each reflection the proportion of the strongly reflected wave-length is
increased until ultimately there is practically only this one
wave-length present. It can therefore be very easily measured. These
waves resulting from a number of successive reflections, rest-strahlen
or residual rays as they have been named, have been very largely used
for investigating long waves. Quartz gives rest-strahlen of length
.00085 centimetres and very feeble ones of .0020 centimetres long.
Sylvite gives the longest rays yet isolated, the wave-length being .006
centimetres.
+Range of the Waves.+--The lengths of the waves thus far measured are:--
Schumann waves . . . . . . . . .00001 to .00002 cms.
Ultraviolet . . . . . . . . . .00002 to .00004 "
Violet . . . . . . . . . . . . .00004 "
Green . . . . . . . . . . . . .00005 "
Red . . . . . . . . . . . . . .00006 to .000075 "
Infra-red . . . . . . . . . . .000075 to about .0001 "
Rest-strahlen from quartz . . .00085 and .0020 "
Rest-strahlen from Sylvite . . .0060 "
Thus the longest waves are six hundred times the length of the shortest.
The corresponding range of wave-lengths of sound would be a little more
than eight octaves, of which the visible part of the spectrum is less
than one.
Electromagnetic Induction.--In the attempt to explain the nature of an
electromagnetic wave (pp. 17-21) it was stated that an electric wave
must always be accompanied by a magnetic wave. In order to {81}
understand the production of these waves, the relation between electric
and magnetic lines of force must be stated in more detail. A large
number of quite simple experiments show that whenever the electric
field at any point is changing, _i.e._ whenever the lines of force are
moving perpendicular to themselves, a magnetic field is produced at the
point, and this magnetic field lasts while the change is taking place.
An exactly similar result is observed when the magnetic field at a
point is changing--an electric field is produced which lasts while the
magnetic field is changing. When the electric field changes,
therefore, there is both an action and a reaction--a magnetic field is
produced and this change in magnetic field produces a corresponding
electric field. This induced electric field is always of such a kind
as to delay the change in the original electric field; if the original
field is becoming weaker the induced field is in the same direction,
thus delaying the weakening, and if the original field is becoming
stronger the induced field is in the opposite direction, thus delaying
the increase.
+Momentum of Moving Electric Field.+--Imagine now a small portion of an
electric field moving at a steady speed; it will produce, owing to its
motion, a steady magnetic field. If now the motion be stopped, the
magnetic field will be destroyed, and the change in the magnetic field
will produce an electric field so as to delay the change, _i.e._ so as
to continue the original motion. The moving electric field thus has
momentum in exactly the same way as a moving mass has. The parallel
between the two {82} is strictly accurate. The mass has energy due to
its motion, and in order to stop the mass this energy must be converted
into some other form of energy and work must therefore be done. The
electric field has energy due to its motion--the energy of the magnetic
field--and therefore to stop the motion of the electric field, the
energy of the magnetic field must be converted into some other form,
and work must therefore be done. One consequence of the momentum of a
moving mass is well illustrated by the pendulum. The bob of the
pendulum is in equilibrium when it is at its lowest point, but when it
is displaced from that point and allowed to swing, it does not swing to
its lowest point and stay there, but is carried beyond that point by
its momentum. The work done in displacing the bob soon brings it to
rest on the other side, and it swings back again only to overshoot the
mark again. The friction in the support of the pendulum and the
resistance of {83} the air to the motion makes each swing a little
smaller than the one before it, so that ultimately the swing will die
down to zero and the pendulum will come to rest at its lowest point.
The graph of the displacement of the bob at different times will
therefore be something like Fig. 28. Should the pendulum be put to
swing, not in air, but in some viscous medium like oil, its vibrations
would be damped down very much more rapidly, and if the medium be
viscous enough the vibrations may be suppressed, altogether, the
pendulum merely sinking to its lowest position.
[Illustration: FIG. 28.]
+Electric Oscillation.+--These conditions have their exact counterpart
in the electric field. To understand them, three properties of lines
of force must be borne in mind: (i.) lines of force act as if in
tension and therefore always tend to shorten as much as possible; (ii.)
the ends of lines of force can move freely on a conductor; (iii.) lines
of force in motion possess momentum. Now imagine two conducting plates
A and B, Fig. 29, charged positively and negatively, and therefore
connected by lines of force as indicated. Let the two plates be
suddenly connected by the wire _w_, so that the ends of the lines of
force may freely slide from A to B or _vice-versa_, and therefore all
the lines will slide upwards along A and B, and then towards each other
along _w_, until they shrink to zero {84} somewhere in _w_. The
condition of equilibrium will evidently be reached when all the lines
have thus shrunk to zero, but the lines which are travelling from A
towards B will have momentum and will therefore overshoot the
equilibrium condition and pass right on to B. That is, the positive
ends of the lines will travel on to B, and similarly the negative ends
will pass on to A. The lines of force between A and B will therefore
be reversed. The tension in the lines will soon bring them to rest,
and they will slide back again, overshoot the mark again, reach a limit
in the original direction and still again slide back. The field
between A and B will therefore be continually reversed, but each time
its value will be a little less, until ultimately the vibrations will
die down to zero. Thus if we were to replace the displacement in Fig.
29 by the value of the field between A and B we should have an exactly
similar graph.
[Illustration: FIG. 29.]
The amount by which the oscillations are damped down will depend upon
the character of the wire _w_. If it is a very poor conductor it will
offer a large resistance to the sliding of the lines along it, and the
vibrations will be quickly damped down or, if the resistance is great
enough, be suppressed altogether.
This rapid alternation of the electric field will send out
electromagnetic waves which die down as the oscillations decrease.
+The Spark Discharge.+--In practice the wire _w_ is not actually used,
but the air itself suddenly becomes a conductor and makes the
connection. When the electric field at a point in the air exceeds a
certain limiting strength, the air seems to break down and {85}
suddenly become a conductor and remains one for a short time. This
breaking down is accompanied by light and heat, and is known as the
spark discharge or electric spark.
+Experiments of Hertz.+--In the brilliant experiments carried out by
Hertz at Karlsruhe between 1886 and 1891, he not only demonstrated the
existence of the waves produced in this way, but he showed that they
are reflected and refracted like ordinary light, he measured their
wave-length and roughly measured their speed, this latter being equal
to the speed of light within the errors of experiment.
[Illustration: FIG. 30.]
One arrangement used by Hertz is shown in plan in Fig. 30. A Ruhmkorff
coil R serves to charge the two conductors A and B until the air breaks
down at the gap G, and a spark passes. Before the spark is {86}
produced, the lines of force on the lower side of AB will in form be
something like the dotted lines in the figure, but as soon as the air
becomes a conductor, the positive ends of the lines will surge from A
towards B and on to B, and the negative ends will surge on to A. These
to and fro surgings will continue for a little while, but will
gradually die out. As the surgings are all up and down AB, the
electric vibrations in the electromagnetic waves sent out {87} will all
be parallel to AB, and therefore they will be polarised.
[Illustration: FIG. 31.]
This is characteristic of all electric waves, as no single sparking
apparatus will produce anything but waves parallel to the spark gap.
The electric vibrations coming up to a conductor placed in the position
of the wire rectangle, M, will cause surging of the lines along it,
and, if these surgings are powerful enough, will cause a spark to pass
across the small gap S.
Such a rectangle was therefore used by Hertz as a detector of the
waves, but since that time many detectors of very much greater
sensitiveness have been devised.
+Reflection.+--In order to show that these waves are reflected in the
same way as light waves, Hertz placed the sparking knobs, G, at the
focus of a large parabolic metallic reflector, and his detector, D, at
the focus of a similar reflector placed as in Fig. 31, but much farther
away (cf. Fig. 1). In this position sparking at G produced strong
sparking in the detector, although the distance was such that no
sparking was produced without the reflectors.
+Refraction.+--The refraction of the waves was {88} shown by means of a
large prism made of pitch. This had an angle of 30° and was about 1.5
metres high and 1.2 metres broad.
[Illustration: FIG. 32.]
Setting it up as shown in plan in Fig. 32, strong sparking was produced
in the detector, thus showing that the rays of electric waves were
deflected by 22° on passing through the prism.
Moving the mirror and detector in either direction from the line LM,
made the sparks decrease rapidly in intensity, so that the exact
position of LM can be determined with considerable definiteness.
+Wave-length, by Stationary Waves.+--The wave-lengths of the
oscillations were found by means of what are known as stationary waves.
When two exactly similar sets of waves are travelling in opposite
directions over the same space, they produce no effects at certain
points called nodes. These nodes are just half a wave-length apart.
Their production can be understood by reference to Fig. 33. The dotted
lines represent the two waves which are travelling in the direction
indicated by the arrows. In A the time is chosen when the waves are
exactly superposed, and the resultant displacement will be represented
by the solid line. The points marked with a cross will be points at
which the displacement is zero.
[Illustration: FIG. 33.]
In B each wave has travelled a distance equal to a quarter of a
wave-length, and it will be seen that the two sets of waves cause equal
and opposite displacements. The resulting displacement is therefore
zero, as indicated by the solid line. In C the waves have travelled
another quarter of a wave-length and {89} are superposed again, but in
this case the displacements will be in the opposite directions from
those in A. In D, still another quarter wave-length has been traversed
by each wave, and another quarter wave-length would bring back the
position A.
In E, we have the successive positions of the wave drawn in one
diagram, and we notice that the points indicated by a cross are always
undisplaced and their distance apart is one-half a wave-length.
Hertz produced these conditions by setting up his coil and sparking
knobs at some distance from a reflecting wall, Fig. 34. Then the waves
which are coming up to the wall and those which are reflected {90} from
the wall will be travelling in opposite directions over the same space.
True, the reflected waves will be rather weaker than the original ones,
so that there will be a little displacement even at the nodes, but
there will be a well-marked minimum. Thus when the detector is placed
at A, B, C or D no sparking or very feeble sparking occurs, while
midway between these points the sparking is very vigorous, and the
distance between two successive minima is one-half a wave-length.
[Illustration: FIG. 34.]
The wave-length will depend upon the size, form, &c., of the conductors
between which the sparking occurs, for the time which the lines of
force take to surge backwards and forwards in the conductors will
depend upon these things. Other things being equal, the smaller the
conductors the smaller the time and therefore the shorter the
wave-length. The shortest wave which Hertz succeeded in producing was
24 centimetres long, but since then waves as little as 6 millimetres
long have been produced.
{91}
The waves which are produced in a modern wireless telegraphy apparatus
are miles in length.
We thus see that there is rather a large gap between the longest heat
waves which have been isolated, .006 cms., and the shortest electric
waves, .6 cms. The surprising fact, however, is that this gap is so
small, for the heat waves are produced by vibrations within a molecule,
or at most within a small group of molecules, whereas the electric
surgings, even in the smallest conductors, take place over many many
millions of molecules.
In conclusion, therefore, we see that from the Schumann waves up to the
longest heat waves a little over eight octaves of electromagnetic waves
have been detected, then after a gap of between five and six octaves
the ordinary electrically produced electromagnetic waves begin and
extend on through an almost indefinite number of octaves.
{92}
BOOKS FOR FURTHER READING
J. H. Poynting, _The Pressure of Light_.
E. Edser, _Heat for Advanced Students_: the chapters on Radiation.
E. Edser, _Light for Advanced Students_: the chapters on the Spectrum.
B. W. Wood, _Physical Optics_: the chapters on Fluorescence and
Phosphorescence, Laws of Radiation, Nature of White Light, and
Absorption of Light.
{93}
INDEX
ABSORBING power, 37
-- and radiating power, 38
Absorption, spectra, 34
-- by glass and quartz, 77
-- by air, 78
Addition of waves, 25
Amplitude, 23
BALMAIN, luminous paint, 59
Boltzmann, laws of radiation, 48
CONVECTION currents, 67
Corpuscular theory, 10
-- reflection and refraction by, 11
Crookes' radiometer, 67
DEWAR, temperature and phosphorescence, 62
Diffraction grating, 72
-- dispersion by, 75
-- wire grating, 77
Dispersion, 29, 75
Doppler effect, 69
EFFICIENCY in lighting, 52
Elastic solid theory, 17
Electric field, 18
Electric charges within the atom, 21
Electric oscillations, 19, 83
Electrification, positive and negative, 18
Electromagnetic induction, 80
Electromagnetic waves, 17, 84
Electrons, 30
Energy in simple wave, 25
Energy--wave-length curve, 27
FLUORESCENCE, 58
-- theory of, 60
Foucault, speed of light in different media, 17
Fourier's series of waves, 26, 30
Fraunhöfer lines, 35
Full radiator and absorber, 44, 45
GASES as radiators, 42
HUYGHENS' wave theory, 13
Hertz, experiments on electric waves, 85
-- reflection, 86
-- refraction, 87
-- wave-length by stationary waves, 88
INFRA-RED rays, 32
Interference, 13
KIRCHOFF'S law, 40
LANGLEY, Bolometer, 32, 48, 49, 77
Lebedew, pressure of light, 65
Lummer and Pringsheim, law of radiation, 48, 50
MAGNETIC oscillations, 20
Maxwell, electromagnetic theory, 17
-- pressure of light, 64
Momentum of moving electric field, 81
NEWTON, dispersion, 29
-- corpuscular theory, 12
-- law of cooling, 46
Nichols, Rubens and, Rest-strahlen, 79
Nicholls and Hull, pressure of light, 64, 68
PFLÜGER, emission from tourmaline, 43
Phase, 22
Phosphorescence, 59
-- chemical theory of, 61
-- temperature and phosphorescence, 62
Planck, energy and wave-length, 51
Polarised light, emission from tourmaline, 42
Pressure of light, prediction of by Maxwell, 64
-- measurement by Lebedew, 65
-- measurement by Nicholls and Hull, 64, 68
-- on the earth, 68
-- on fine dust, 69
-- on comets' tails, 69
-- three effects of in astronomy, 70
Prévost, Theory of Exchanges, 46
RADIATING power, 38
Radiometer action, 67
Reflection, corpuscular theory, 11
-- of electric waves, 87
Refraction, corpuscular theory, 11
-- of electric waves, 87
Resonance, 30
Rest-strahlen or residual rays, 79
Ripples on mercury, 13
Ritchie, radiating and absorbing powers, 38
Rowland, gratings, 73
Rubens and Kurlbaum, proof of Planck's law, 51
Rubens and Nichols, Rest-strahlen, 79
SCHUMANN waves, 78
Simple harmonic motion, simple periodic motion, 24
Spark discharge, 84
Spectrometer, 76
-- reflecting, 78
Spectrum, 29
-- the whole, 32
-- incandescent solid or liquid, 33
-- incandescent gas, 33
-- analysis, 34
-- emission and absorption, 34
-- sun, 35
-- stars and nebulæ, 36
-- and temperature, 48
Stationary waves, 88
Stefan, law of radiation, 47
TEMPERATURE, absolute, 56
-- of planets, 54
-- of space, 55
-- of sun, 53
ULTRAVIOLET rays, 32, 77
WAVE form, 24
Wave-length, 22
-- range of, 80
-- of electric waves, 90
Wave theory, rectilinear propagation, 13
Wien, Law of Radiation, 50
YOUNG, interference, 16
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summary of events and prospects."--_Daily News_.
51. +SHAKESPEARE+. By Professor C. H. HERFORD, Litt.D.
"Well worth a place alongside Professor Raleigh's book in the 'English
Men of Letters.' ... Sets a high note and retains it without
effort."--_Observer_.
53. +PURE GOLD--A CHOICE OF LYRICS AND SONNETS+. By H. C. O'NEILL.
"An anthology of good poetry such as we might expect from a man of
taste."--_Daily News_.
57. _DANTE_. By A. G. FERRERS HOWELL.
"It is a fine piece of scholarship, and should be read by any one who
is beginning the study of Dante, or indeed any one who is interested
generally in the early process of European literature, for the process
is here admirably analysed."--_The Manchester Guardian_.
THE SECOND DOZEN VOLUMES (Ready)
1. +THE FOUNDATIONS OF SCIENCE+. By W. C. D. WHETHAM, M.A., F.R.S.
11. +INORGANIC CHEMISTRY+. By Professor K. C. C. BALY, F.R.S.
14. +RADIATION+. By P. PHILLIPS, D.Sc.
22. +LORD KELVIN+. By A. RUSSELL, M.A., D.Sc., M.I.E.E.
23. +HUXLEY+. By Professor G. LEIGHTON, M.D.
36. +THE GROWTH OF FREEDOM+. By H. W. NEVINSON.
41. +JULIUS CAESAR: SOLDIER, STATESMAN, EMPEROR+. By HILARY HARDINGE.
43. +ENGLAND IN THE MIDDLE AGES+. By Mrs. E. O'NEILL, M.A.
54. +FRANCIS BACON+. By Professor A. R. SKEMP, M.A.
55. +THE BRONTËS+. By Miss FLORA MASSON.
60. +A DICTIONARY OF SYNONYMS+. By AUSTIN K. GRAY, B.A.
61. +HOME RULE+. By L. G. REDMOND HOWARD.
_List of other Volumes in Preparation may be had_.
LONDON AND EDINBURGH: T. C. & E. C. JACK
NEW YORK: DODGE PUBLISHING CO.