225 
1916-17.] On some Nuclei of Cloudy Condensation. 
drawn up to the stop, when a fairly dense condensation appears in the flask. 
Time is given for the cloud to settle, but if it is dense it will not all settle, 
much of it evaporating. When it has ceased falling, the handle of the 
expander is put back to 0 and time allowed for the air to get saturated, 
when another slow expansion is made and another shower falls. This 
process of slow, large expansions is continued till the condensation gets thin. 
The number of expansions required to secure this is noted and entered in 
the third column of the table. When the condensation becomes thin there is 
a risk of the high expansion giving too high a supersaturation. The stop 
should therefore be put back to a 2 per cent, expansion, and quick ex- 
pansions at this rate continued till the showers cease, the number of 
these 2 per cent, expansions being entered in the fourth column. 
By the above process all the nuclei in the sample of air which were 
active with a slight expansion have been cleared out. We then proceed to 
test if there are any nuclei too small to respond to a 2 per cent, expansion. 
The stop is now moved back to give 1 or 2 per cent, greater expansion. In 
these tests an advance of 2 per cent, was adopted. Setting the slide to give 
a 4 per cent, expansion, a quick motion is given to the handle, and if there 
are any small nuclei a shower will fall. The process is repeated till the 
showers stop, after which the expansion is increased by 2 per cent, and 
the treatment continued till all condensation ceases, and the number 
of showers at each expansion are entered in columns 5, 6, and 7. Of 
course, the expansion must always be kept under 25 per cent., as ions 
become active with that degree of expansion. 
Again referring to Table II, it will be seen that the air at Falkirk generally 
required a number of large, slow expansions at 8 per cent, and a number at 
2 per cent, to clear the air of the larger particles. After the large particles 
were cleared out there were always sufficient smaller particles to give showers 
at 4 per cent., and in some cases at 6 per cent. The number of these small 
particles varied with the weather and the direction of the wind. From a 
great number of tests, not here recorded, it was found that there were 
always fewer particles with southerly than with easterly winds, and that the 
easterly air contained smaller particles than the southerly. We will refer 
to this point later, but it may be stated here that the greater number and 
smaller size of the nuclei in the easterly air has no connection with the 
direction of the wind, but is due to the position of the place of observation 
relative^ to the local pollution. 
In one respect I was unfortunate in not getting any pure air at Loch 
Awe during this visit. The wind never blew from the N.W. quadrant, the 
only direction which brings air with 100 to 200 particles per c.c. On this 
VOL. xxxvii. 15 
