Marcu 31, 1904 | 
resinous lustre. The specific gravity was 4-98. An analysis 
by Mr. Blake furnished the following results :— 
Per cent. 
Thorium oxide = ee =) eLhOs, 66°26 
Cerium oxide (and Cerium earths) ... CeO, 7:18 
Zirconium oxide ZrO, 2°23 
Uranium oxide UO, 0°46 
Ferric oxide Fe,O, 1°71 
Calcium oxide ... CaO 0°35 
Phosphoric oxide P.O; 1°20 
Silica Be SiO, 14°10 
Water H,O 6°40 
99°89 
This mineral is therefore thorite, consisting chiefly of 
thorium silicate. Both these minerals are under further 
investigation at the Imperial Institute. Careful explorations 
are now being made as to the extent of their occurrence 
in Ceylon. 
It is obvious that apart from the scientific interest attach- 
ing to the determination of their composition, the discovery 
in Ceylon of two minerals rich in thoria, now so largely 
employed for the manufacture of incandescent gas mantles, 
may be of considerable commercial importance. 
Imperial Institute, March 29. WynpbuamM Dunstan. 
Ionisation of Air. 
SOME experiments have been recently made at the 
Cavendish Laboratory which seem to throw light on the 
question of the ‘‘ spontaneous’’ ionisation of air. The 
anticipation of a detailed report of these in a short summary 
of the results obtained may serve some useful purpose by 
preventing a waste of energy on the part of others who 
are engaged in investigating the same subject. 
The experiments consist in the determination of the 
saturation current through rectangular vessels, lined with 
the metal under investigation, the volume of the vessels 
being capable of alteration by the motion parallel to itself 
of one of the sides of the vessel. On plotting a curve the 
ordinates of which are the saturations currents and the 
abscissee the distance of the movable side from the side 
opposite to it, it becomes clear that there are two separate 
distinct kinds of radiation causing the ionisation of the 
gas :—(1) a radiation coming from the sides of the vessel 
which is completely absorbed by some 5 cm. of air, and 
which, therefore, when the volume is considerable, gives 
an ionisation proportional to the surface of the vessel; (2) 
a much more penetrating radiation, which at all volumes 
gives an ionisation proportional to the volume of the vessel. 
Further experiments were then made by surrounding the 
vessel with lead sheets about 3 cm. thick and repeating 
the determination of the variation of the ionisation with 
the volume. The lead screen diminished the ionisation ; 
by this method it was possible to discover which part of 
the radiation suffered diminution. 
Up to the present time four metals have been investi- 
gated, lead, aluminium, zine and tin foil. Of these, in 
the absence of the screen, the first three gave approximately 
the same value for the penetrating radiation causing 
volume ionisation. The absorbable radiation causing sur- 
face ionisation was greater for the aluminium than for the 
zinc, and still greater for the lead. When the screen was 
applied the penetrating radiation was diminished to about 
two-fifths of its value for all three metals. In the lead 
and the aluminium the value of the surface ionisation re- 
mained unaltered by the screen, but in the zinc this was 
decreased, and fell to about three-fifths of its original 
magnitude. 
The tin was quite peculiar in its behaviour. The normal 
volume ionisation was only about one-third of that in the 
other metals, and when the screen was applied both the 
surface and the volume ionisations fell in the same pro- 
portion to two-thirds of their former values. 
It is pretty clear, therefore, that at least in the case of 
tin and zinc we have secondary radiation given off from 
the surfaces of those metals under the influence of pene- 
trating radiation coming from outside. 
Some numbers may be useful to give an idea of the 
respective magnitudes of the radiations mentioned. Taking 
NO. 1796, VOL. 69] 
NATURE 
iar 
i : 3 eae: 
an arbitrary unit, the values for the ionisation caused by 
one square centimetre of surface of the metals are as 
follows :—lead 38-6, tin 33, aluminium 10, zinc 7-9. On 
the same scale the values of the ionisations due to the 
penetrating radiation in b c.c. of air enclosed in a vessel 
of these metals is for lead, aluminium and zinc between 
3-2 and 2-8; for tin it is 0-9. 
It is probable that many of the discrepancies that have 
appeared between the results obtained by different physicists 
may be explicable by a difference in the metal of which 
their vessels were composed. For example, it is clear that 
it might be possible to detect the effect of a screen on a 
zine vessel, while in a lead vessel the diminution of 
ionisation due to the same screen would be inappreciable ; 
similarly, it would be possible to measure in a lead vessel 
effects due to the surface radiation which could not possibly 
be detected if zinc were substituted fer the lead. Further 
experiments on different metals, and with other modifica- 
tions, are in preparation, which it is hoped will throw more 
light on this interesting problem. 
Norman N. CAMpBELt. 
Trinity College, Cambridge, March 25. 
Respiration in Frogs. 
Is the buccal cavity of the frog a respiratory chamber? 
In a letter to Nature, March 24, Mr. M. D. Hill accepts 
this conception of it, and yet the only evidence which can 
be offered in support of this view is the rich blood supply 
of its lining membrane. The lungs and skin, which are 
known to be respiratory surfaces, are supplied by a special 
circulation; the buccal cavity is neither more nor less sup- 
plied with blood than the other parts of the alimentary 
tract, which are certainly not respiratory. 
The oscillatory movement of the frog’s pharynx, which 
occurs when the lungs are filled and the opening to the 
larynx closed, is one of a number of points connected with 
the respiratory system which have not yet been satis- 
factorily explained. The other points are :—(1) the evolution 
of the reptilian method of respiration from the amphibian ; 
(2) the meaning of the laryngeal and bronchial musculature 
found in amphibians, reptiles, birds and mammals; (3) the 
closure of the auditus laryngis of the amphibian during 
the respiratory phase; (4) the. attachment of part of the 
transversalis and rectus abdominis to the pericardium and 
roots of the lungs; (5) the air in contact with the re- 
spiratory surface of the lungs is always very impure. All 
these points, with the exception of the last, find their ex- 
planation in the fact that the act of respiration in all 
forms of vertebrate life produces two effects within the 
lungs :—(1) air is drawn into the air spaces; (2) blood is 
drawn into the pulmonary capillaries. Further, the rate 
of flow in the pulmonary capillaries, which are situated in 
the septa between the air cells, is determined by the pressure 
within the air cells. The air within the lung is used as a 
brake for regulating the pulmonary flow of blood. That 
is to say, the act of respiration in reptiles, mammals and 
birds has two effects, one on the air and another on the 
blood within the lung. In amphibians these two effects are 
apparently obtained by separate means. 
In the major movement of amphibian respiration the air 
is forced within the lungs by the muscles of the pharynx 
and expelled by the contraction of the muscles of the body 
wall. In both phases of that movement, which are for the 
renewal of air within the lung, the pulmonary circulation 
is retarded by the positive pressure of the breathed air. 
When the lungs are filled and the opening of the larynx 
closed, the minor movements set in. They vary in different 
genera of frogs, but taking the noisy frog (Rana clamata) 
as a type in which to observe these movements, it will be 
noticed that the body wall muscles, especially the trans- 
versalis, contract and rather expand the body at the same 
time as the larynx is drawn downwards. In all Amphibia 
the larynx, pharynx, and their muscles are so closely bound 
up with the lung that the pressure of the pulmonary air 
must be affected by their movement. In short, the 
oscillatory movements of the pharynx in the Amphibia (and 
also in turtles and tortoises) create a negative pressure 
' within the amphibian lung, and thus regulate and accelerate 
the flow of blood through that organ. For that reason 
