IN THE PRESENCE OF DUST-FREE AIR AND OTHER GASES. 
■275 
If the expansion be slow, the supersa.turation can nowhere greatly exceed that 
required for the formation of the first drops. 
With very sudden expansions, however, even if they be much greater tlian that 
required to produce rain-like condensation, the drops which are the first to begin to 
form will not have time to grow sensibly before the expansion is completed, and 
their influence on the temperature and vapour contents of the air will be confined 
to a very small region round each. In that case the lowest temperature and 
maximum supersaturation reached throughout the greater part of the moist air will 
be the same as that calculated on the assumption that no condensation takes place. 
The more numerous the drops, the shorter must the time taken in expansion be 
made, in order that there should be no sensible error due to the formation of the 
drops commencing before the expansion is completed. 
Now the time of expansion can be made shorter in a small machine. The new 
expansion apparatus was therefore made upon quite a small scale, the effect of the 
reduced dimensions in increasing the error due to the walls being counterbalanced by 
the great reduction in the time of expansion. 
The expansion ought evidently to be made most rapid just before it is completed, 
since it is just in the later stages of the expansion that drops are being formed, and 
we wish to reduce, as far as possible, their chance of growing appreciably before the 
expansion is completed. This end was kept in view in designing the apparatus. 
The expansion apparatus (fig. 2), is made wholly of glass, to reduce the risk of 
contamination of the gas under investigation. This is contained in the space A 
under a pressure of from 20 to 40 centims. of mercury above that of the atmosphere. 
This expansion chamber A, is bounded below by the hollow-glass piston P, which is 
ground down so that it just slides freely in the outer tube. 
There is, as indicated in the figure, an annular constriction on the latter. Into 
this the lower end of the piston has been ground with fine em.ery, so that, with no 
other lubricant than water, it prevents the gas in A escaping, even when the excess 
of pressure above is half an atmosphere or more. 
The lower end of the tube is conical, with a circular aperture about 1 centim. 
in diameter, closed by a glass plug G. The grinding here, too, has to be sufficiently 
thorough to enable a pressure of two atmospheres to be maintained above it for 
several minutes without leakage, with only one or two drops of water to serve as 
lubricant. 
The upper end of the tube is drawn out and joined to a narrow-bore tube provided 
with a stopcock T^, serving for the introduction of the gas and water. 
When the apparatus is in use, the inner surface of A is covered with a film of 
water, which also fills the narrow space left between the piston and the walls of the 
tube. 
By pumping air by means of the mercury pump on the left, into the space C below 
the piston, we can drive up the latter and so compress the gas in A to any extent 
2 N 2 
