270 
tion, and change inclination. Such a_ straight 
segregation of clean water shows that no self- 
attraction of the suspended solids can be the 
cause. In a recent. point of view it looks 
like a liquid crystal arrangement of water expelling 
the powder as foreign matter, especially when we- 
remember the habit of ice crystals in very thin plates. 
The question then arises, Are all these emulsion 
figures due to the clear liquid segregating and expell- 
ing the suspended matter ? 
‘ W. M. Fiinpers PETRIE. 
—— 
MODERN FORMS OF RONTGEN-RAY 
TUBES. 
Ls spite of many obstacles, medical technique 
in the application of Réntgen rays has made 
steady progress during recent years. But there 
still remain certain primary difficulties which are 
often a source of hindrance and confusion. The 
demand for a more trustworthy method of work- 
ing than exists to-day is the natural outcome of 
wider radio-therapeutic experience, but what is 
required above all is an accurate means of measur- 
ing the output of the tube. 
The solution of this problem clearly requires 
that we shall have at our disposal an apparatus 
capable .of emitting a specific type of ray in 
definite quantity; and were it not for some ex- | 
periments by Dr. Lilienfeld, and more recently | 
by Mr. Coolidge, of New York, there would be 
little prospect of actually realising this result in 
practice. I shall refer to their work more in 
detail later. 
Meanwhile, it is worth noticing that the modern 
X-ray tube, with all its imperfections, is a tri- 
umph: of craftsmanship. It is the result of 
numberless costly and difficult experiments carried 
out by manufacturers and others to meet a demand 
which grows more exacting every year. The 
collection of historical X-ray tubes brought 
together some time ago by the R6ntgen Society, 
and now on view in the Science Museum at South 
Kensington, contains the first bulb which em- 
bodied the chief features adopted universally up 
to the present time in the construction of X-ray 
tubes. That bulb was made by Prof. Herbert 
Jackson in 1896, and measures only 3 in. across. 
The diameter of those in use to-day is, however, 
usually twice or three times as great, the elec- 
trodes being heavy and the vacuum carefully 
adjusted. 
The successful working of the apparatus de- 
pends so largely upon this last factor that decrease 
in pressure of the residual gas, invariably accom- 
panying prolonged use, has to be compensated 
for. The devices employed for regulating the 
vacuum may be divided into three main classes, 
viz., those :— 
(1) In which a few discs of mica, a piece of 
carbon or asbestos, etc., fixed within the bulb, 
offer an alternative path for the discharge, so 
that gas is, when necessary, liberated by the heat 
generated, as the electric current follows the line 
of least resistance. 
(2) In which a fine palladium tube stopped up 
at one end has a small tubular extension of 
NO. 2324, VOL. 93] 
NATURE 
| tive. 
[May 14, 1914 
platinum soldered to it for sealing into the X-ray 
bulb. A gas flame brought near so as to heat the 
palladium enables hydrogen to enter by osmosis, 
and so lower the vacuum. All the tubes used 
in the X-ray treatment cubicles at the London 
Hospital, for instance, have these “Osmo” regu-: 
lators. 
(3) In which air is allowed to enter through the 
pores of a piece of unglazed porcelain, which is 
usually sealed with mercury until by a pneumatic 
contrivance it is momentarily uncovered. 
But none of these methods is free from objec- 
tions. The regulation is generally too insensi- 
The tube often outlives the supply of gas 
from the first sort, and the others are only trust- 
worthy in skilled hands. The mica disc regulator 
is shown in the diagram (Fig. 1), which other- 
wise is self-explanatory. 
Messrs. C. H. F. Muller describe some eighteen 
types of tubes of this character in their recently 
Water Bulb--...---}._... 
Waterslubes 2 oe y. --- -.---Anode Cap 
Saecno-ete Anode 
Seed ica Regulator 
CS) Mica Regul 
AEN »-------Carbon Regulator 
(..._Regulator Cap 
Biihee 
Antikathode____ 
Marget—— £5 
Kathode Head_______- 
Kathode Neck_..._--_- 
Kathode Stem____.._.._- 
~-------Regulator Wire 
Karhode Cap 
Fic. 1.—Showing the terms in common use to denote the different 
parts of the instrument. 
published catalogue, and give precise instructions 
for the adjustment and use of each kind. There 
are in addition five coloured plates showing the 
appearance of X-ray bulbs in action, and a great 
amount of useful information besides. Fig. 2 
(Muller) may be taken as representing a typical 
example of modern heavy discharge X-ray tube. 
The cause of the disappearance of gas with pro- 
longed use has given rise to much speculation. 
It has been suggested that the ions produced by 
the discharge are driven actually through the 
walls, and so escape; there is proof at least that 
the glass takes up a large part of the residual 
gas under these conditions, and that lead glass 
absorbs more than Jena. 
Since the degree of vacuum controls the resist- 
ance of the tube, and this in turn determines the 
current that passes with a given potential differ- 
ence between the electrodes, it is evident that 
the pressure of the residual gas is the chief factor 
which defines the type and quantity of rays to be 
