476 
Proceedings of the Royal Society of Edinburgh. [S'ess. 
this be so, then I is unscrewed and the stop J lowered to give a greater 
compression. When this is done it will be found that at a certain 
compression a few drops appear in B. If we go on pushing J down by 
stages so as gradually to increase the compression, it will be found that the 
number of drops increases. These drops are formed on some nuclei which 
Wilson has shown are always present in moist air. These nuclei for 
convenience we will call natural nuclei. If we go on increasing the 
compression it will be found that when it exceeds a certain amount that the 
condensation somewhat suddenly changes its appearance from the rainy 
form to the foggy, and this fog becomes denser with further increase in 
the compression. 
In the apparatus shown in fig. 1 we cannot tell without trial what 
amount of compression will be necessary to give the supersaturation 
required to cause condensation on the natural nuclei, as the action of the 
instrument is complicated. Part of the expansion in that form of apparatus 
is due to the compression of the ball D, but part is also due to this expansion 
being carried on by the impetus given by the expanding air to the water in 
the tube. That is, some of the energy put into the water at the beginning 
of the expansion is given out by it at the end. The result of this is that the 
compression necessary for this form of apparatus is less than that which 
Wilson found to be necessary with his apparatus for causing condensation 
in the absence of dust. As has been stated, Wilson found that when the 
expansion was as great as vjv x — 1*250 that condensation took place in 
dustless air, and for all his instruments this figure was fairly constant, but 
in the instrument above described this is far from being the case. Any 
slight alteration is the arrangement of the apparatus alters the degree of 
compression required. For instance, any change in the amount of water in 
the U tube alters the compression necessary, and any alteration in the tube 
connecting the U tube with the compression ball has a like effect. In 
Wilson’s apparatus a compression corresponding to a column of about 19 cm. 
of mercury would, when expanded, give the necessary supersaturation, but 
with the U tube a pressure of only from 14 to 16 cm. is required according 
to the conditions. In Wilson’s apparatus the foggy stage of the condensa- 
tion is arrived at when the compression is about equal to 28 ’8 cm., while 
the U tube requires only from 19 to 20’5 cm. It is impossible to give these 
pressures with greater definiteness, as they vary in all the instruments here 
described with but slight alterations in their arrangement, but the pressures 
remain constant while the conditions are constant. 
Other forms of the compression chamber were tried. The one shown at P, 
fig. 1, for instance, requires much greater compression to cause condensation 
