Borings in a Coral Reef 255 



very great measure, of high frequency, belonging to the 

 ultra-violet part of the spectrum. But if thoria be nearly 

 transparent for that part, the etherial vibrations will mostly 

 pass through without disturbing it, and thus fail to produce 

 the very high agitation desired. But when the thoria is 

 mixed with a small quantity of a suitable oxide, such as 

 that of cerium or of uranium, which is opaque for rays 

 of high refrangibility, their molecules take up agitation 

 from the ether and pass it on to the thoria by molecular 

 conduction. If a large quantity of the oxide were mixed 

 with the thoria it would be injurious, because then it would 

 freely emit radiations of comparatively low frequency, and 

 so a great quantity of the agitation, which it takes up from 

 the ether disturbed by the combustion of the gas would be 

 wasted on etherial vibrations of a kind which are not wanted. 

 But when thoria is agitated by the so-called cathodic rays, 

 that is, by molecular bombardment, the process is wholly 

 different and the foreign oxide is not required. 



1900. Feb. 8th, 474th meeting. Professor Edgeworth 

 David, of the University of Sydney (a guest), gave an account 

 of the boring which had been put down in the atoll of 

 Funafuti (see page 253) to a depth of 1114 feet, and described 

 his own stay on the island in 1897, while the first part of 

 that bore-hole was being made. 1 In this last expedition 

 two borings, near together, had been made in the bed of 

 the lagoon by the aid of Captain Sturdee, one to a depth of 

 144 feet, the other of 94^ feet. The result of the three 

 expeditions was to obtain maps and a study of the geography, 

 geology, botany, and zoology of the atoll, and make a complete 

 section of it, which passed partly through coral rock, partly 

 through calcareous sand, and the materials obtained had 

 been sent for study to the British Museum. He considered 

 that the evidence tended to show that at Funafuti and other 

 atolls in the Ellice group, algae, like Halimeda and Litho- 

 tkamnion, were not less important than corals and foramini- 

 fera in their construction, and that the existence of large 



1 An interesting account of this visit was written by Mrs. Edgeworth 

 David in Funafuti, or Three Months on a Coral Island, 1899. 



