J. D. Dana— Oceanic Coral Island Subsidence. 31 
absorption of moisture from the atmosphere by the sulphuric 
acid produces the dew-like deposits observed. 
The deleterious effect upon the insulators can be remedied 
by neutralizing the acid with some substance which will not 
form a hygroscopic compound or essentially lessen the insulat- 
ing power of the ebonite. I have used oxide and carbonate 
of magnesium with very good effect. A little of either of 
these substances in fine powder is sprinkled upon a soft cloth 
or piece of chamois leather and rubbed over the insulators. 
T € excess is removed with a wet cloth, and the surface, after 
drying, cleaned and polished by rubbing with a soft woolen 
cloth very slightly moistened with carbonic di-sulphide. As 
the ebonite is attacked by the latter substance, care should be 
served, in employing it, to use only so much as is needed to 
facilitate the polishing process without injuring the surface. 
The ebonite may be somewhat discolored by these operations, 
but the color cah be restored by rubbing with a little oil, or 
will return of itself after a time. 
good results. On one occasion, early last autumn, when the 
electro-machine had not been used for some months, the sparks 
under-water prolongation, longer than that above water: the 
line of the Hawaiian Islands, for example, which has a am ce" of 
Only four hundred miles from Hawaii to Kauai, and five 
hundred and thirty to Bird Island, the western rocky islet of 
the group, but stretches on westward, as the coral registers show, 
* From the closing chapter of the writer's work on Corals and Coral Islands, 
(Pp. 364-372), recently published by Dodd & Mead, New York. 
