In communications to this Society in 1893 and in 1912, and in other papers, I have shown that some of the nuclei in the atmosphere have a power of condensing water vapour even when the air is unsaturated, owing to the material of which they are composed having an affinity for water. All kinds of nuclei have an affinity for water vapour, and by their surface action condense some water at all degrees of humidity, so causing increase in their size and hazing effect; but the particles having an affinity for water condense very much more and cause fogs in. unsaturated air. In the present communication I intend to deal with the more common form of nuclei—that is, those which have little or no chemical affinity for water—and with those nuclei which require supersaturation to cause condensation; the reason for this investigation being some statements which have appeared in scientific journals in this country and abroad regarding the nuclei counted by the condensation method. It has been definitely stated that the greater number of the nuclei counted are not dust particles but only ions, or aggregations of them. It therefore seemed desirable that further investigation should be made into the subject to see if there was anything to support this ionic theory of the condensation nuclei in the air, and at the same time to inquire into their origin.