1888.] 



The Chemical Composition of Pearls. 



461 



about for some time by means of their cilia before they attach them- 

 selves to a suitable spot, and undergo their further development. 

 These planulse are necessarily swept away by strong currents to the 

 further shores of a reef, and it has been shown experimentally by 

 von Koch that they will not attach themselves in a strong current, or 

 if attached, will loose their hold when a strong current is directed 

 upon them. Although coral colonies grow larger by budding, they 

 originate in every case from planulse, and no great group of corals 

 could grow in a place where the strong currents prevented planulas 

 from attaching themselves. — March 20]. 



EXPLANATION OF PLATE. 



FlG. 1 represents a diagrammatic section through the surface soil of East Islet 



showing the alternate layers of sand and rock of which it is composed. 



Scale J inch to the foot. 

 FiG. 2 shows the way in which a current striking a sloping bank is deflected upwards 



over its surface until it joins the superficial part of the main current at the 



upper edge of the reef. 



III. " The Chemical Composition of Pearls." By George 

 Harley, M.D., F.R.S., and Harald S. Harley. Received 

 February 23, 1888. 



Although there are many qualitative analyses of pearls, from our 

 being unable, in their voluminous literature, to find any evidence 

 of a quantitative analysis of their ingredients having been recorded, 

 we undertook the examination of several varieties, of which the fol- 

 lowing is an account : — 



1st. As regards oyster pearls. Of these three varieties were 

 examined, British, Australian, and Ceylonese. 



The qualitative analyses showed that they all had an identical com- 

 position, and that they consisted solely of water, organic matter, and 

 calcium carbonate. There was a total absence of magnesia and of all 

 the other mineral ingredients of sea- water — from which the inorganic 

 part of pearls must of course be obtained. Seeing that ordinary 

 sea-water contains close upon ten and a half times more calcium 

 sulphate than calcium carbonate, one might have expected that at 

 least some sulphates would have been found along with the car- 

 bonates, more especially if they are the mere fortuitous concretions 

 some persons imagine them to be — a view we cannot endorse, from the 

 fact that by steeping pearls in a weak aqueous solution of nitric acid, 

 we are able to completely remove from them all their mineral con- 

 stituents without in any way altering their shape, and but very 

 slightly changing their naked eye appearances, so long as they are 



