432 
MR. C. T. R. WILSON ON THE CONDENSATION NUCLEI PRODUCED IN 
side and the metal under investigation on the other, or simply of metal polished on 
one side only, or of two metals, such as zinc and copj^er, in contact. 
Another and better form of apparatus which was used in some of the later experi¬ 
ments is that shown in fig. 10b. The top of the tube (2’8 centims. in diameter) was 
closed by a metal plate, bolted down, with an indiaruhber band interposed as a 
washer. Half of the lower surface of the metal plate was covered with a semicircular 
sheet of mica, attached to the metal by a little shellac; a vertical mica partition 
divided the apparatus into two equal parts; the roof of the one compartment being 
of mica, that of the other of the metal whose effect was to he investigated. 
The apparatus first described had this defect—that when a fog had been produced 
Fig. 10a. Fig. 10b. 
by expansion, the metal caused the re-evaporation of the drops in the air near it, 
before they had time to settle to the bottom of the vessel. This made the process of 
removing dust particles, by repeatedly forming a cloud by expansion and allowing 
it to settle, a very slow one in an apparatus of this kind. With the metal at the top 
there is no difficulty of this kind, for any drops formed near it very quickly fall out 
of reach of the metal. 
In no case were the metals found to produce nuclei requiring only slight expansion 
to catch them. 
This simplified the method of working, making it possible to remove all ordinary 
dust particles originally present, or any droj)s remaining in suspension after a cloud 
had been produced, by the expansion method just referred to, without any arrange¬ 
ment for shielding the air from the action of the metal while this was being done. 
When this process had been completed, so that with expansions of moderate amount 
drops were no longer produced, the expansion might be increased without any visible 
condensation generally to the point = 1’25) where a few drops are produced 
even in the absence of any metal. 
When, however, the expansion was such that a rain-like condensation would have 
resulted in the absence of any metal {v-i/ih being between I'25 and 1’38), the number 
