1909-10.] The Clay Substance in the Grlenboig Fireclay. 377 
This decomposition by hydrochloric acid goes far to prove that the 
mineral is halloysite ; for, according to Lacroix,* kaolinite is not affected by 
concentrated hydrochloric acid, whereas halloysite is readily attacked. 
Since the time taken to decompose the clay substance in the Glenboig fire- 
clay is considerable, it can scarcely be said to be readily attacked. The 
greater resistance to decomposition may be found to depend on the much 
smaller content of loosely combined water which it contains. With a view 
to comparing the behaviour of other recognised halloysites towards hydro- 
chloric acid, two samples were treated in exactly the same manner. A 
halloysite from the Dordogne yielded 2*2 per cent, of A1 2 0 3 after two hours’ 
boiling, while a sample of lenzinite from the Eifel, Rhine, gave 21*97 per 
cent, in the same time. It is interesting to note that the Dordogne halloy- 
site contains only 4 per cent, of loosely held water, while lenzinite has 8*8 
per cent. 
The evidence in favour of the mineral being halloysite may be summed 
up : 
1. The percentage of water is higher than that required for kaolinite. 
2. Although the percentage of water is rather lower than that in the 
typical halloysites, it must be remembered that a very great variation is 
shown in their analyses. 
3. The mineral is practically completely decomposed by boiling with 
concentrated hydrochloric acid. 
* Mineralogie de la France , vol. i., 1895, p. 472. 
(Issued separately April 20, 1910.) 
