1910-11.] On some Nuclei of Cloudy Condensation. 
483 
The Physics and Chemistry of the Sun-formed Nuclei. 
Turning now to the physics and chemistry of these sun-formed nuclei, 
one naturally wishes to know something about their origin — what is the 
material out of which they are made, and what is their nature. There are 
three substances which suggest themselves as the possible origin of these 
nuclei. These are, first, the salt formed in the drying sea-weed ; second, 
iodine, which is supposed to be present in the air on the sea-shore ; and 
third, hydrochloric acid, another substance generally admitted to be present. 
Experimental work was begun to see if any of these substances produced 
nuclei under the influence of sunshine. 
The method of testing is shown in fig. 1. S is a flask in which the 
substance to be tested is placed and exposed to sunshine. This flask is 
connected by a tube with a stopcock on it with the flask T, in which is 
placed some water and its sides kept wet. T is connected with the air-pump 
P. For convenience we will call the flask T the test flask, as the air to 
be tested for nuclei is drawn into it and saturated with water vapour, and 
afterwards expanded by the pump P, and the density of the condensation 
noted. These experiments should be made in a darkened room, only 
as much light being admitted as is necessary for sunning the flask. 
A black background to the test flask also helps to make the observa- 
tions accurate and easy. For illuminating the test flask a gas jet, and 
sometimes an incandescent burner L, has been used, the light being placed 
in the dark lantern D, which is provided with a vertical opening so 
arranged that it can be made of the most suitable width ; and between this 
opening and the test flask T is placed a condenser C in the form of a 
globular glass flask filled with water. This condenser is a great con- 
venience, as it absorbs the radiant heat and prevents the test flask from 
getting heated on the side next the light. This unequal heating often 
gives rise to what looks like spontaneous condensation, though it is only due 
to the hot moist air rising from the heated glass and mixing with the colder 
air. Further, by preventing the heating of the side of the test flask it is not 
necessary to turn it upside down so frequently in order to wet the sides. 
At first it was proposed to use unfiltered air so as to interfere as little 
as possible with the conditions in nature. It was hoped that, by noting the 
difference in the density of the condensation with the sunning flask S 
empty and with the substance under test in it and sunned, definite 
conclusions might be obtained. This, however, was not found practicable, 
as the purity of the air was not constant, and it was impossible to 
separate the alterations in density from this cause from those due to any- 
