﻿Pro/! Liversidge — Chalk in New Britain. 



531 



The specific gravity was taken from a mass weigliing about 

 78 grammes, which was allowed to soak in water for about one hour 

 and a half, in fact until all air-bubbles ceased to be evolved ; a small 

 quantity of the block scaled off when immersed in the water — a 

 correction for which had to be made. 



The above figures show that in round numbers about 81 per cent, 

 of the specimen consists of calcium . carbonate ; thus it is undoubtedly 

 a far less pure limestone than the ordinary white chalk, as the follow- 

 ing figures indicate. 



Chemical Composition of Chalk. 



A specimen of chalk, from near Grravesend, which was analysed by 

 Mr. W. J. "Ward, yielded the following results : — 



Calcium carbonate ... 98-52 



Magnesium carbonate -29 



Calcium sulpbate '14 



Manganese binoxide -04 



Phosphoric acid ... traces 



Organic matter 



Insoluble matter, chiefly silica 



•65 



99-64 



Mr. David Forbes, F.R.S., also examined some specimens of chalk, 

 the analyses of which are here' cited ; the first analysis shows the com- 

 position of a piece of white chalk from Shoreham, in Sussex ; and the 

 second of a piece of grey chalk from Folkestone. 



White chalk. 



Calcium carbonate ... 98-40 . 



Magnesium carbonate 

 Phosphoric acid | 

 Alumina and loss ) '"' 



Sodium chloride 



Water 



Insoluble rock debris 



•08 

 •42 



Grey chalk. 

 . 94-09 

 -31 



trace 



1-10 



100-00 



1-29 



•70 



3-61 



100-00 



(Vide "Geology of England and Wales," by H. B. Woodward, p. 239.) 

 Another sample of chalk obtained from a well at Driffield was found 

 by Mr. T. Hodgson to have the following composition : — 



Moisture 520 



Calcium carbonate ^ 



Magnesium carbonate 



Iron sesquioxide and alumina 

 Silica 



93-30 



•15 



•20 



M5 



100^00 



It is, however, far less impure than the "chalk mud" of the 

 Atlantic, for the analysis quoted by Professor Sir Charles Wyville 

 Thomson, F.E.S., in his "Depths of the Sea," p. 469, shows that the 

 " chalk mud " contains merely some 60 per cent, of calcium carbonate, 

 and with as much as from 20 to 30 per cent, of silica, and varying 

 proportions of alumina, magnesia, iron, and other substances. The same 

 author mentions that the typical chalk is free from silica, and so it 

 would appear to be from the above-quoted analyses ; but the " insoluble 



