May 18, 1899] 
NATURE 6a 
April 27.—‘*On the Luminosity of the Rare Earths when 
heated in vacuo by means of Kathode Rays.” By A. A. 
Campbell Swinton. Communicated to the Royal Society by 
Lord Kelvin, F.R.S. 
For incandescent gas mantles it is found that certain definite 
mixtures of the rare earths are necessary in order to obtain the 
maximum luminosity. For instance, a mantle consisting of 
pure thoria or pure ceria will in the Bunsen flame only give about 
one-eleventh of the light of one composed of 99 per cent of 
thoria and 1 per cent of ceria, which is the mixture used by the 
Welsbach Company. 
In order to explain this remarkable fact, several contradictory 
theories have been propounded, and with a view to elucidating 
matters the author has made experiments in which mantles 
composed of different pure oxides and mixtures were heated by 
kathode ray bombardment in vacuo. 
The mantles were prepared according to the ordinary 
Welsbach process, and in order to obtain accurate comparisons 
the mantles were made in patchwork, each complete mantle 
being made up of two or four sections separately impregnated 
with different solutions. The mantles were so mounted in the 
vacuum tube that the kathode rays impinged equally upon the 
portions that consisted of different oxides and mixtures, so that 
an equal amount of energy was imparted to each sample. 
Under these conditions the Welsbach mixture of thoria plus 
I per cent. of ceria was found to give very little more light than 
pure thoria, the difference probably not exceeding 5 per cent., 
but on starting the kathode discharge the mixture heated up to 
incandescence more rapidly, and on stopping the discharge 
cooled more rapidly than the pure thoria. At the same time it 
was found that with an intensity of kathode rays that gave a 
brilliant light both with pure thoria and with the Welsbach 
mixture, a mixture of 50 per cent. thoria and 50 per cent. ceria, 
and also a piece of mantle composed of pure ceria, gave prac- 
tically no light, becoming barely red-hot. 
The maximum luminosities could only be obtained at a critical 
and highly unstable degree of vacuum, which rendered accurate 
photometrical measurements impossible, but with pure thoria 
the amount of light under favourable conditions was estimated 
at at least 150 candle-power per square inch of incandescent 
surface, this being obtained with an expenditure of electric 
energy at about $000 volts pressure of approximately one Watt 
per candle. : 
The kathode rays were found to havea reducing action on the 
oxides, which became discoloured under the bombardment, the 
discoloration disappearing owing to re-oxidation on the ad- 
mission of a small quantity of air. Air so admitted while the 
tube was working was rapidly absorbed, and after the process of 
admitting air and absorbing it had been repeated several times, 
the degree of exhaustion which gave the maximum incan- 
descence was found to have altered considerably, the residual 
gas having apparently become less conducting. 
In place of air, oxygen and hydrogen were separately used as 
the residual gas, but without any difference in the luminosity. 
These experiments show that thoria and ceria, both alone and 
mixed, behave quite differently when heated by kathode ray 
bombardment than when heated in a Bunsen flame. In the 
latter thoria plus 1 per cent. of ceria gives many times as much 
light as pure thoria alone, while when incandesced by kathode 
rays of equal intensity the difference, though in a similar 
direction, is only just appreciable. Again, in the flame, pure 
ceria gives just about the same amount of light as pure thoria, 
while with a given intensity of kathode ray bombardment thoria 
gives a brilliant light, while ceria gives practically none. In 
arriving at any satisfactory theory of the luminescent properties 
of the rare earths, these results will have to be taken into 
account. 
**A Quartz Thread Gravity Balance.” 
J. A. Pollock. 
The balance is of the horizontal, stretched, quartz thread 
type. One end of the thread is attached by soldering to a 
spring of peculiar construction ; the other end is attached to the 
axle of the vernier arm of a sextant. At the centre of the 
thread a bit of brass wire is attached by soldering, so that the 
thread crosses the wire, which is about two cm. long, at right 
angles. The centre of gravity of the bit of wire, which will 
be referred to as the ‘‘lever,” lies a little to one side of the 
thread, so that when the thread is untwisted the lever hangs 
vertically. The thread is stretched so that, in spite of the weight 
of the lever, it hangs almost horizontally. To make this 
NO. 1542, VOL. 60] 
By R. Threlfall and 
arrangement into a gravity balance, it is only necessary to turn 
the lever round the thread as axis, so that each half of the 
latter receives about three turns (3 x 360 degrees) of twist. 
The lever is adjusted till, under these circumstances, it hangs 
nearly horizontally. A discussion of the theory of the balance 
shows that if the twist be now reduced the centre of gravity of 
the lever will rise and the position of the lever become unstable 
soon after its centre of gravity rises above the horizontal plane 
through the thread. The nearly horizontal position of the lever 
is secured during observation by means of a microscope. which 
can be focussed upon the end of the lever, and which is rigidly 
attached to the framework of the instrument. Gravitational at- 
traction on the lever is thus balanced by the torsional rigidity 
of the quartz fibre, and the observations consist in noting the 
increase or diminution of twist, as applied at one end of the 
thread, necessary to bring the lever to its sighted position. The 
whole apparatus is enclosed in a tube which is air-tight, the 
vernier axle working through a sort of mercury stuffng-box. 
Exact thermometry is required, and is supplied by means of a 
platinum thermometer lying alongside the thread. 
The instrument only gives relative values of gravity, referring 
an excess, or defect, of gravitational force to the difference of 
gravitational intensity at two stations selected as having known 
constants, in the present case Sydney and Melbourne. 
The difficulties which have been met with during many years? 
work arise from the warping of the metallic parts of the in- 
strument under changes of temperature and in the imperfect 
elastic properties of fused quartz threads. 
The possible errors of a single observation are shown, from a 
discussion of the detail of the instrument, to amount to about one 
part in 300,000 of the value of g al any point, and by a dis- 
cussion of three journeys ‘between Sydney’ and Hornsby 
(N.S.W.), it is shown that the consistency actually realised is 
about one in 500,000 of g. ; 
Many journeys have been made with the instrument in New 
South Wales, Victoria, and Tasmania, from which the perfect 
portability of the instrument has been ascertained, as well as its 
convenience in practice.’ A single observation takes only a few 
minutes after the temperature has arrived at a maximum or 
minimum, but the packing and unpacking occupy more than 
an hour—in general about three hours are required. The weight 
of the total outfit, with ordinary appliances’ just as they came 
to hand in the laboratory, is 226 pounds, but this might be 
halved by making the appliances specially. The paper contains 
the complete theory of the instrument, working drawings ex- 
hibiting its construction, and an account‘ of experiments made 
with various modifications of the instrument. 
‘*On the Electrical Conductivity of Flames containing Salt 
Vapours.”. By Harold A. Wilson, B.Sc. (Lond. and Vic.), 
1851 Exhibition Scholar. Communicated by Prof. J. J. 
Thomson, F.R.S. 
’ The experiments described in this paper were undertakem, 
with the object of following up the analogy between the con- 
ductivity of salt vapours and that of Rontgenised gases, and‘ 
especially of getting some information about the velocities of the 
ions in the flame itself. : 
They are to some exent a continuation of the research of 
which an abstract has already been published in the Proceedings 
of the Royal Society {‘*The Electrical Conductivity and 
Luminosity of Flames containing Vaporised Salts,’ by A. 
Smithells, H. M. Dawson, and H. A. Wilson, Roy. Soc. Proc., 
vol. Ixiv. p. 142). 
The paper is divided into the following sections :— 
(1) Description of the apparatus for producing the flame. 
(2) The relation between the current and E.M.F. in the 
flame. 
(3) The fall of potential between the electrodes, 
(4) The ionisation of the salt vapour. 
(5) The relative velocities of the ions in the flame. 
(6) The relative velocities of the ions in hot air. 
(7) Conclusion. 
The current with a large E.M.F, was found to be independent 
of the distance between the electrodes in the flame, provided 
both were hot enough to glow ; it was much greater when the 
hotter electrode was negative than when it was positive. When 
both electrodes were hot, the fall of potential between them 
was found to be very like that observed in the discharge through 
gases at low pressure. If one of the electrodes was cool, then 
nearly all the fall of potential occurred very near toit. Practically 
all the ionisation of the salt vapours appeared to take place at 
