Aggregations produced in Gases by Sudden Cooling. 751 
again into the gaseous condition. In order to settle this 
point, further experiments have since been carried out 
on C0 2 . 
The gas was prepared in two ways. In the first method 
it was obtained by heating pure sodium bicarbonate. It was 
then passed through calcium chloride, P 2 5 , a tight plug of 
cotton-vool, and finally solidified in a glass tube surrounded 
by a tall Dewar vessel containing liquid air. When a 
quantity of C0 2 was required the liquid-air vessel was lowered, 
and after the desired amount had sublimed the flow of gas 
was checked by replacing the Dewar vessel. The tube 
containing the solid C0 2 formed therefore a very convenient 
gas-holder of large capacity. On its way from this gas- 
holder to the apparatus the gas parsed through a plug of 
cotton-wool. 
In the second method, the gas was obtained direct from 
the steel tubes in which it is supplied commercially and 
purified and condensed as before. The C0 2 obtained by 
these two methods gave the same results. 
It was found that the gas could be cooled to any tempera- 
ture not lower than its condensing point, both by the 
"sudden" and "slow" methods, without a single nucleus 
being produced. (By the "sudden" method the gas could 
be cooled with certainty to within a degree of its condensing 
point.) As the expansions used for the detection of the 
nuclei (where not otherwise stated) were about 1'10 some 
expansions nearly large enough to catch the ions in C0 2 
were tried, but with the same result. This proves that for 
C0 2 (in contradistinction to air) no nuclei at all are pro- 
duced when the gas is suddenly cooled right down to its 
condensing point. 
It was found, however, that when condensation actually took 
place and the condensed gas was allowed to sublime, nuclei 
were present and their number increased with the amount 
of C0 2 which had been condensed. The nuclei obtained when 
C0 2 sublimes are considerably bigger than those obtained by 
cooling air or oxygen. The slightest supersaturation in the 
expansion apparatus caused by the adjusting of the pressure- 
drop preparatory to the expansion, brings down large numbers, 
but if care be taken to obviate the slightest fall in pressure, 
no drops are seen until an expansion is actually made. This 
means that the nuclei produced on the sublimation of the 
solid gas are bigger than those produced in air, but the fact 
that a certain degree of supersaturation (though small) is 
needed to initiate condensation on them proves them to be 
entirely different in character from that class of " chemical 
