622 
NATIORE 
[APRIL 29, 1897 
and high resistance in the tube, and least penetrative with low 
electrical power and low resistance. 
The author has further found that it is possible to vary the 
penetrative value of the N-rays produced by a focus tube, by 
simply altering the distance between the kathode and anti- 
kathode. 
Fig. 1 shows such a tube, in which the anti-kathode B of 
aluminium, faced with platinum, is connected to the anode 
terminal by a sliding steel rod, so that it can be moved along 
the axis of the tube; and the distance between the kathode C 
and the anti-kathode B varied from 1 to 3 inches. With such 
a tube, exhausted so as to give X-rays of medium penetrative 
value, with the anti-kathode midway between its two extreme 
positions, if the anti-kathode be now placed nearer to the 
above mentioned, produce X-rays of high penetrative value, 
are those that would conduce to a high average velocity of the 
molecules at the moment at which they strike the anti-kathode, 
and at the same time toa high average difference of potential 
between the travelling molecules and the anti-kathode at the 
moment of impact. Conversely the conditions which produce 
X-rays of low penetrative value, are such as would conduce to a 
lower average velocity of the molecules, and to a less difference 
of potential between the latter and the anti-kathode. 
Further, since some molecules will strike the anti-kathode 
at higher velocities and in a more highly-charged state than 
others, the same hypothesis will account for X-rays being more 
or less hydrogenous under all conditions. 
Finally, it appears that the penetrative value, as distinct from 
kathode, the X-rays immediately become of a higher pene- 
trative value, just as though the vacuum had been increased ; 
while again, when the anti-kathode is moved in the opposite 
direction, and placed at a greater distance from the kathode, the 
X-rays become less penetrative, and similar to those produced 
at a lower vacuum. In this way, without varying the vacuum, 
the penetrative value of the X-rays can be increased or decreased 
within limits, as desired. 
Again, the author has found that the penetrative value of the | 
X-rays can be altered by employing kathodes of different 
diameters. In the tube shown in Fig. 2, 
kathodes, C and C’, both focussing upon opposite sides of the 
same anti-kathode B. The kathodes are of dissimilar diameter, 
C’ being about twice the diameter of C. Ifa tube of this type 
be exhausted to the degree necessary to give rays of medium 
I 
penetrative value with the smaller kathode, and the connections 
are then altered so as to bring the larger kathode into operation, 
the X-rays are immediately found to have become of very low 
penetrative value. If the vacuum be then increased so that, 
with the larger kathode in use, X-rays of medium penetrative 
value are obtained, it will then be found that with the smaller 
kathode X-rays of much higher penetrative value are pro- 
duced. 
In all the above-mentioned experiments the author has found | 
that the conditions which produce X-rays of high penetrative 
value are also the conditions that produce a considerable 
potential difference between the anode and kathode portions 
of the tube, and at the same time a high electrical excitation 
of the kathode. 
On the assumption that kathode rays consist of negatively 
charged molecules that are repelled from the similarly electrified 
kathode, with an initial velocity that depends upon the degree 
there are two | 
the quantity of X-rays, is independent of the material of which 
the anti-kathode surface is made. The author has made experi- 
ments with tubes fitted with anti-kathodes of aluminium, iron, 
copper, silver, and platinum, and finds that though the metals 
of high atomic weight form the most efficient anti-kathodes, 
and give a greater quantity of X-rays, as measured by photo- 
graphic action, or by the brightness of a flurorescent screen, all 
these metals appear to give X-rays of the same penetrative value 
under similar conditions. 
** Condensation of Water Vapour in the presence of Dust-free 
Air and other Gases.”’ By C. T. R. Wilson, M. A., Clerk-Maxwell 
Student in the University of Cambridge. Received March 15. 
The apparatus used enabled an exceedingly rapid expansion 
of any desired amount to be effected. The following is a sum- 
mary of the results obtained. 
If air, initially saturated and free’ from foreign nuclei, be 
allowed to expand suddenly, a rainlike condensation results if 
the ratio of the final to the initial volume 7,/z, exceeds 1°252 ; 
no condensation taking place except on the walls of the vessel 
with smaller expansions. 
The number of the drops produced remains small if 7/2, does 
not exceed 1°37. Beyond this point the number increases at an 
exceedingly rapid rate with increasing expansion, the cloudlike 
condensation which then results showing colour phenomena of a 
very definite kind. 
In the presence of air, oxygen, nitrogen, or carbonic acid, 
rainlike condensation results when the expansion is sufficient to 
cause the supersaturation to exceed a certain limit amounting 
when the final temperature is - 6° C. to between 4"2 and 4°4, and 
diminishing with rising temperature. 
By the supersaturation is here meant the ratio of the actual 
density of the vapour when the expansion has just been com- 
pleted, and the minimum temperature has, therefore, been 
reached to the density of the vapour in equilibrium over a flat 
surface of water at that temperature. 
The condensation is cloudlike in the presence of any of these 
gases or of hydrogen, when the expansion is sufficient to cause 
the supersaturation to exceed a certain value, amounting, when 
the final temperature is— 16° C. to about 7°9. F 
When the supersaturation reached lies between these limits 
rainlike condensation results in all these gases, except hydrogen, 
in which scarcely any trace of condensation is seen when the 
supersaturation is even slightly below 7°9. 
A statement of the effect of the Rontgen rays on condensation 
in the presence of air has already been published. They cause 
a great increase in the number of the drops produced, the mini- 
mum expansion required to cause condensation being, however, 
unaltered. When hydrogen is substituted for air their effect is 
similar, the nuclei introduced by the action of the rays requir- 
ing the supersaturation to reach the same limit as is required for 
rainlike condensation in air, in order that condensation may take 
place upon them. 
Chemical Society, March 25.—Mr. A. G. Vernon Har- 
court, President, in the chair.—Prof. P. Frankland delivered 
of electrical excitation of the kathode, the conditions which, as | the Pasteur Memorial Lecture (see p. 518). 
NO. 1435, VOL. 55] 
