Marcu 18, 1897 | 
NAT 
URE Hah 
true that with some of the new forms of circuit breakers and 
storage batteries, an induction coil can be made to work fairly 
well during prolonged runs ; but storage batteries are troublesome, 
and a break that will work on voltages ordinarily supplied for 
lighting is yet to be made. 
The following method of driving an induction coil not only 
does away with its former disadvantages, but gives a much more 
powerful means of exciting X-ray tubes. A condenser of con- 
siderable capacity is first charged by connecting its terminals 
to the ordinary lighting mains ; it is then disconnected, and dis- 
charged through the primary of an induction coil. Any good 
induction coil can be used in this way with a single change, viz. 
anew primary. The primary should be a few turns of heavy 
wire on a finely laminated core, 
A six-inch Ritchie vertical coil with a new primary winding 
of about thirty turns of heavy wire (6 B.S. gauge) gives, when 
a condenser of 27 micro-farads charged at 220 volts is discharged 
through its primary, a long thin zig-zag spark, resembling that 
from a static machine with smallcondensers. If now some form 
of rotary commutator be used to charge and discharge the con- 
densers, and this be run at sufficient speed, a continuous dis- 
charge of sparks will take the place of the single discharge at 
the secondary terminals. 
The commutator used has six segments. If this is run at 
2000 revolutions per minute by a small fan motor, there will be 
12,000 discharges through the coil per minute, or 200 per second. 
At this speed there is a continuous discharge of zig-zag sparks 
a little over six inches long. We have not yet run the com- 
mutator above 2000 revolutions, but there is no indication that 
we are near the limit of speed. Sparking on the commutator 
is slight, and the power taken from the mains is but a few 
amperes. 
It is necessary to have the primary of the coil well insulated, 
not only from the secondary, but its own turns must be well 
insulated from one another and the core. An easy and effectual 
way in the case of a vertical coil is to place the laminated core | 
in a glass tube, upon this wind the primary, then place the 
whole in a large heavy tube closed at the bottom, and fill with | 
oil. Without insulation there is a tremendous brush discharge 
within the primary. Undoubtedly the efficiency of the coil 
would be considerably increased by using an oil insulated 
secondary, but it is questionable whether the gain would be 
enough to off-set the trouble and dirt of oil insulation. 
An X-ray tube of the focus type and proper resistance con- | 
nected to the terminals of the coil lights up brilliantly, and with 
a spark gap in series, the length of which seems to make very | 
little difference, shows no indication whatever of anything but a | 
unidirectional discharge through the tube. 
Fluorescent screens become brilliantly illuminated. In a 
darkened room all the bones of the hand and forearm can be 
distinctly seen on a calcium tungstate screen at a distance of 
eight feet from the tube. The penetration seems to be unusually 
strong. The whole of the trunk can be examined with the 
greatest ease with the tube several feet distant. The hand can | 
be distinctly seen through the abdomen, the most opaque part 
of the body. 
Photographically, X-rays obtained in this way are no less 
powerful. Excellent fully-timed photographs of the hand can 
be taken in twenty-five seconds with the tube twelve inches 
from the plate, photographs not merely showing the outline of 
the bones, but showing the details of the bones, the finger-nails, 
tendons, &c. Forty-five seconds is an over-exposure. One of 
the best photographs of a small object—a pocket-book—we have 
seen, was taken in less than a second. 
It seems apparent that we have a simple method for exciting 
X-ray tubes that is far more powerful and efficient than any 
that has yet been used. It isa method that ought to be par- 
ticularly adapted to the needs of the physician, and requires no 
more skill or knowledge of physics than the ordinary practitioner 
can supply. CHARLES L. Norton. 
2 RavpH R. LAWRENCE. 
Rogers Laboratory of Physics, Mass. Inst. Technology, 
Boston, February 20. 
Semi-Permeable Films and Osmotic Pressure. 
Lorp KELVIN’s very interesting problem concerning mole- | 
cules which differ only in their power of passing a diaphragm | 
(see NATURE for January 21, p. 272), seems only to require for | 
its solution the relation between density and pressure for the | 
NO. 1429, VOL. 55] | 
fluid at the temperature of the experiment, when this relation 
for small densities becomes that of an ideal gas; in other cases, 
a single numerical constant in addition to the relation between 
density and pressure is sufficient. 
This will, perhaps, appear most readily if we imagine each of 
the vessels A and B connected witha vertical column of the 
fluid which it contains, these columns extending upwards until 
the state of an ideal gas is reached. The equilibrium which we 
suppose to subsist will not be disturbed by communications 
between the columns at as many levels as we choose, if these 
communications are always made through the same kind of 
semi-permeable diaphragm as that which separates the vessels 
Aand B. It will be observed that the difference of level at 
which any same pressure is found in the two columns is a 
constant quantity, easily determined in the upper parts (where 
the fluids are in the ideal gaseous state) as a function of the 
composition of the fluid in the A-column, and giving at once 
the height above the vessel A, where in the A-column we find 
a pressure equal to that in the vessel B. 
In fact, we have in either column 
dp = — gyaz, 
where the letters denote respectively pressure, force of gravity, 
density, and vertical elevation. If we set 
== F(A), 
y 
we have 
F'(p)dp = — gaz. 
Integrating, with a different constant for each column, we get 
F(f,) = ~g(2- Ca) 
F( fs) = —g(=— Cz) 
F(f,) — F( ps) = (Ci — Ce). 
In the upper regions, 
a 
== 
oy 
-. F(s) =at log 4, 
where ¢ denotes temperature, and @ the constant of the law of 
Boyle and Charles. Hence, 
at log p, — at log pz = g{C, — Cy). 
| Moreover, if I : 72 represents the constant ratio in which the S- 
and D-molecules are mixed in the A-column, we shall have in 
the upper regions, where the S-molecules have the same density 
in the two columns, 
¥. = (1 + 2) 7s pr=(I + n)pe 
e(C, — Cy) = at log (1 + 2). 
Therefore, at any height, 
F(f.) — F(fs) = a¢ log (1 + 2). 
| This equation gives the required relation between the pressures 
in Aand B and the composition of the fluidin A. It agrees 
with van *t Hoff’s law, for when z is small the equation may 
| be written 
F'(f,)( fa — pu) = ain 
Pa — ps = anys. 
But we must not suppose, in any literal sense, that this differ- 
ence of pressure represents the part of the pressure in A which 
is exerted by the D-molecules, for that would make the total 
pressure calculable by the law of Boyle and Charles. 
To show that the case is substantially the same, at least for 
any one temperature, when the fluid is not volatile, we may 
suppose that we have many kinds of molecules, A, B, C, &c., 
which are identical in all properties except in regard to passing 
diaphragms. Let us imagine a row of vertical cylinders or tubes 
closed at bothends. Let the first contain A-molecules sufficient 
to give the pressure f’ at a certain level. Then let it be con- 
nected with the second cylinder through a diaphragm imper- 
meable to B-molecules, freely permeable to all others. Let the 
second cylinder contain such quantities of A- and B-molecules 
as to be in equilibrium with the first cylinder, and to have a 
or 
| certain pressure f” at the level of ’ in the first cylinder. Ata 
higher level this second cylinder will have the pressure which 
we have called g’. There let it be connected with the third 
cylinder through a diaphragm impermeable to C-molecules, and 
to them alone. Let this third cylinder contain such quantities 
of A-, B-, and C-molecules as to be in equilibrium with the 
