GASES BY THE ACTIOX OF EONTGEX RAYS AND OTHER AGENTS. 
417 
Dense fogs were obtained under the action of the ultra-violet light (zinc-spark) ; 
the expansion being that corresponding to a pressure fall of 10 centiins. 
The nuclei produced under these conditions cannot have arisen from the disinte¬ 
gration of the quartz. 
Another proof that the quartz is not the source of the nuclei is furnished by 
the results of experiments made with an expansion apparatus of the form shown 
in fig. 3. It consisted of three glass tubes meeting at right angles, two being 
horizontal and the third pointing downwards and containing the piston. A quartz 
window was fixed in a vertical j^lane making an angle of 45° with each of the hori¬ 
zontal arms. By placing the zinc points forming the source of the ultra-violet light 
at a position such as A, the rays may he made to pass along only one limb of the 
apparatus. If the source is transferred to B the air in the otlier liml) is exposed to 
the rays and none of them traverse the first limb. The quartz being equally inclined 
to both limbs, any nuclei which are thrown oft' from it will find their way e([ually 
readily along either. The experiments show, however, that nuclei are only introduced 
into that limb along which the idtra-violet light passes. 
Tlie fogs were made visible l)y the light from a gas ftame, which could l^e con¬ 
centrated by a condensing lens at any part of either tube. The time for which the 
rays were allowed to act before the expansion was made was generally 5 seconds. In 
some of the experiments the rays were made approximately parallel by means of a 
quartz lens. The expansions used, measured by the pressure fallp, varied from 13 to 
16 centiins. of mercury. 
In every case a shower of fog was produced from end to end of the tube traversed 
by the ultra-violet rays, while no effect could he detected in the other branch even at 
a point not more than 1 centim. from the junction of the tubes. The exposure could 
be made twice as long without any effect being ofitained in the branch not exposed 
to the rays. 
In all these experiments attention was confined to the small portion of one tube 
which was illuminated by the light from the luminous gas flame, which was lirought 
to a focus at that point; observations being made alternately with corresponding 
portions of the two branches successively illuminated in this way. Finally, however, 
experiments were made, in which, owing to the use of stronger radiation this was 
unnecessary, the fogs produced being well seen without any condensing lens. The 
sparks were produced between cadmium terminals, and a more powerful induction 
coil was used than in the previous experiments. A parallel beam of ultra-violet 
light was not used, but the cadmium points were brought to within 1'5 centiins. from 
the quartz plate. The experiments were made with a pressure fall of 14 centiins., 
the time of exposure being 20 seconds. Under these conditions, the tube along 
which the rays were directed was filled with fog on expansion, the other tube 
remaining empty. The fogs were well seen by means of the light from the gas 
flame without any condensing lens, so that a general view of the result of expansion 
YOL. CXCII.-A. 3 H 
