NUCLEAR CONDENSATION OF CERTAIN ORGANIC VAPOURS. 
455 
Summary of Least Expansions for Condensation of Vapours in Air acted upon 
by Rontgen Rays. 
Vapour. 
Expansion. 
Vapour. 
Expansion. 
Ethyl acetate .... 
1-486 
Formic acid .... 
. 1-782 
Methyl butyrate . 
1-334 
Acetic acid. 
. 1-441 
Methyl iso-butyrate . 
1-347 
Propionic acid .... 
. 1-343 
Propyl acetate .... 
1-310 
n-butyric acid .... 
. 1-380 
Ethyl propionate . 
1-410 
Iso-butyric acid . 
. 1-360 
Iso-amyl alcohol 
1-182 
Iso-valeric acid 
. 1-220 
No very regular relationship 
can be seen between the constitution 
of the above 
vapours and the least expansion 
necessary 
to produce condensation in 
air saturated 
with them. The expansion, with some exceptions, decreases with increase of molecular 
weight. On calculating, however, the supersaturation, which exists in the vapour at 
the moment before condensation takes place, more regular relationships become 
evident. 
Calculation of Supersaturation necessary for Condensation. 
The calculation of the supersaturation which exists before condensation takes place 
has been made for gases saturated with water vapour by Mr. Wilson.* When air 
expands adiabatically, the final temperature is given by the relation 
OJOi = (vi/v 2 ) y -\ .( 1 ) 
where v x and v 2 are the initial and final volumes, 6 l and d 2 the initial and final 
temperatures of the air, and y the ratio of its specific heats ; y = D404. 
We will assume (the assumption will he discussed later) that this relation gives the 
final temperature of the mixture of air and vapour in a condensation experiment.! 
This d 2 then becomes the temperature of the air and vapour at the end of the expansion 
before any condensation lias taken place. 
The supersaturation, S, is conveniently defined as the ratio of the pressure of vapour 
* ‘Phil. Trans.,’ A, 189, p. 265, 1897. 
f Instantaneous Temperature by a Platinum Thermometer .—When about half way through this investigation, 
the writer thought of using a platinum thermometer (after the manner Callendar determines cylinder 
temperatures) to find the lowest instantaneous temperature produced by an expansion. There appears to 
be, however, an unsurmountable difficulty. A slight supersaturation would cause condensation on the 
thermometer wire itself, and the latent heat of condensation would heat the wire. I have seen such 
condensation actually take place on a hair in the apparatus. A stream of drops fell from it when the 
expansion was made. The presence of the wire would alter the conditions of the experiment. It certainly 
would bp far more satisfactory to determine the lowest temperatures directly if it were possible. 
