193 
1911-12.] The Sun as a Fog Producer. 
It was thought it might he interesting to see to what distance this very 
rapid diffusion took place. The above experiment was made in a globular 
flask 8 cm. diameter, and, as stated, the whole cloud disappeared in a second. 
A flask 13 cm. diameter was now tried. When the condensation was made 
in this flask all the cloud did not disappear at once, but a small ball of cloud 
was left in the centre of the flask. This small cloud fell quickly in a mass 
to the bottom, where it rapidly disappeared. The fall of the cloud being 
due to the friction of the cloud particles on the air giving it a quasi-greater 
density than the surrounding air, its fall was much quicker than that of 
the small drops through the air, and the changed shape of the cloud on 
striking the water showed it had moved in a mass. 
A saturated solution of salt being unsuitable for these tests on the 
condensing power of different nuclei, experiments were then made with 
weaker solutions. If 1 part of saturated solution was mixed with 19 parts 
of water in the test-flask, it was found to meet the requirements, there being 
no spontaneous condensation with ordinary air, even with slight inequalities 
of temperature. This will be partly due to the air not being saturated, but 
part will be due to the salt on the sides of the flask checking evaporation, 
even when slightly heated owing to increased concentration of the salt by 
loss of vapour. 
Some measurements were made on the effect of the salt in lowering the 
vapour tension. Two similar flasks about 10 cm. diameter were used. In 
one was placed a concentrated solution of salt, and in the other some tap 
water. In each of the flasks was hung a wet-bulb thermometer. The wet- 
bulb in the flask with tap water was required to act as a control of the 
reading of the thermometer in the flask with salt water. The result was 
that rhe evaporation caused by the salt kept the temperature of its wet- 
bulb 3*5° lower than the other. As the vapour in the flask had to diffuse 
some distance to the salt water, it was thought that if the salt water was 
nearer the bulb the difference in the readings might be greater. Accordingly, 
two similar cylindrical vessels, about 3*5 cm. diameter and 18 cm. deep, 
were lined with blotting-paper. One of the cylinders contained the 
saturated solution of salt, the other only water. Both cylinders were 
plunged in a vessel of water to keep the temperature as steady as possible. 
The result was that the wet-bulb surrounded by salt fell only 3'6° below 
the other, or but a fraction of a degree lower than when larger vessels were 
used, a result which was indicated by the rapid disappearance of the cloud 
particles in 10-cm. flasks previously described. It should be mentioned 
that for these tests two exactly similar thermometers were used, having 
equal-sized bulbs and equally covered with muslin ; and also that the 
VOL. xxxii. • 13 
