1882.] On a Collection of Hock Specimens from Socotra. 145 



straight length of fine wire. One end of the wire was connected by 

 platinnm foil with the liqnid in an insnlated glass bottle, from which 

 one of the jets was fed. The glass bottle supplying the second nozzle 

 was similarly connected with a moveable point on the stretched wire. 

 The electromotive force necessary to cause union, as measured by the 

 distance between the two fine wire contacts, though definite at any one 

 moment, was found to vary on different occasions, possibly in conse- 

 quence of forces having their seat at the surfaces of the platinum 

 oil. From one-half to three-quarters of the whole force of the 

 Daniell was usually required. 



With a view to further speculation upon this subject, an important 

 question suggests itself as to whether or not there is electrical 

 contact between colliding and rebounding jets. To solve this question 

 it was only necessary to introduce a fine wire reflecting galvano- 

 meter into the arrangement just described, taking care that the 

 electromotive forces employed fell short of what would be required 

 to cause the union of the jets. Suitable keys were introduced for 

 more convenient manipulation, and sulphuric acid was added to 

 the water, in order to make sure that absence of strong galvanometer 

 deflection could not be due merely to the high resistance of the thin 

 columns of water composing the jets. Repeated trials under these 

 conditions proved that so long as the jets rebounded their electrical 

 insulation from one another was practically perfect. 



As to the explanation of the action of electricity in promoting 

 union, it would be possible to ascribe'it to the additional pressure called 

 into play by electrical attraction of the opposed water-surfaces, acting 

 as plates of a condenser. But it appears much more natural to regard 

 it as due rather to actual disruptive discharge, by which the sepa- 

 rating skin is perforated, and the equilibrium of the capillary forces 

 is upset. A small electromotive force, incapable of overcoming the 

 insulation of the thin separating layer, is without effect. 



XIV. " On a Collection of Rock Specimens from Socotra." By 

 Professor T. G. BoNNEY, M.A., F.R.S., F.G.S. Received 

 June 12, 1882. 



(Abstract.) 



In the spring of 1879 the island of Socotra, which lies off the 

 north-east corner of Africa, about 140 miles from Cape Gardafui, was 

 visited by Professor Bayley Balfour. Landing at the north-west 

 extremity, he traversed the northern side of the island up to the 

 eastern end, then returning by a more central course to the sea, he 



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