192 Proceedings of the Royal Society of Edinburgh. [Sess. 
form any cloudy condensation without expansion. For this examination 
the magnifying lens will be necessary, as the cloud particles so formed 
are often very small. This is a somewhat delicate experiment, because any 
inequality in the temperature of the sides of the flask, or of the water in 
it, will give rise to mixtures of airs of different temperatures and humidities, 
which by mixing will cause supersaturation and condensation on nuclei 
which have no affinity for water vapour. It is therefore necessary to 
prevent all unequal heating of the test-flask, and the light should be as 
carefully filtered from heat rays as possible, and only allowed to enter 
the flask for as short a time as is necessary for examination. A better 
plan, however, is to use the light of the sky, if there is nothing in the flask 
on which it can act. For making these tests it has been found convenient 
to surround the flask with a small box painted black inside, with a small 
opening about 1 or 2 cm. diameter for admitting the light, and a larger 
one for seeing what is taking place in the flask. 
With all these precautions, one is never quite certain that any condensa- 
tion taking place in the flask where there is no expansion may not be due 
to inequalities of temperature. It therefore seemed desirable to make some 
tests in air that was not quite saturated with water vapour. At first it was 
thought that the simplest method of doing this would be to mix a certain 
proportion of dry air with the saturated air. But as the humidity of the air 
of the laboratory, as well as its temperature, is a variable quantity, constant 
degrees of dryness could not be depended on without the use of complicated 
apparatus. It was then thought that if a little chloride of sodium were 
i 
dissolved in the water in the test-flask, its presence would lower the vapour- 
pressure in the flask ; and if it was found that the presence of the solution 
of this salt in the flask had no effect of itself, or on the gases tested, its use 
was permissible. A saturated solution was first tried, but it was found to 
reduce the vapour pressure too low for anything but powerful fog-producers 
making themselves visible. The effect of the presence of the salt on ordinary 
condensation produced by expansion was so interesting that it may be 
referred to here. The vapour-pressure of a saturated solution is not too 
low to prevent the formation of a dense cloud in ordinary air, if sufficient 
expansion be made ; but the interesting point was that the cloud did not 
remain so long a time as usual, but rapidly vanished, being gone in a second. 
The affinity of the salt for water is so great that it robbed the nuclei of their 
water, which evaporated, diffused, and was absorbed by the salt solution. 
The change took place with marvellous rapidity, pointing to the rate of 
diffusion of water vapour in air being very great ; and this experiment is a 
simple method of illustrating this. 
