Mr. A. B. Northcote on the Constitution of Allophane. 339 



altogether unprofitably so, since by the examiuation of a larger 

 number of samples I hope I have been enabled to obtain some 

 insight into the constitution of this curious silicate. 



The specimens which I have examined are four in number : 

 the two first taken in the summer of 1856 at the opposite sides 

 of the pit, the two latter taken about six months later from one 

 of the sides, in the immediate neighbourhood of each other. 

 Their physical characters were somewhat different : No. I. was 

 an amber-coloured transparent substance, bearing a strong resem- 

 blance to the gum-resin copal; No, II. was semi-opake, and 

 more friable than the former ; it had also a fainter tinge of yel- 

 low, and occurred in larger masses ; No. Ill, bore a considerable 

 likeness to No. II. ; and No. IV. was of a ruby-rect colour, and 

 transparent, and appeared to be generally disposed in the form 

 of layers, alternating with the ordinary allophane. The speci- 

 mens were all found to contain, besides alumina, silica and water, 

 lime, carbonic acid, and more or less iron ; and these constitu- 

 ents were estimated according to the usual methods. Taking 

 into consideration, howevei', the fact of the large and very vari- 

 able quantity of water present in all the varieties of which ana- 

 lyses have been published, and by means of which such difficulties 

 have been thrown in the way of arriving at any formula for the 

 mineral, I considered it desirable accurately to ascertain the 

 amount of loss which each specimen experienced by exposure to 

 a heat of 100° C, in order to see whether anymore definite view 

 of their constitution might be obtained by knowing the compo- 

 sition of the dried substances. The loss thus undergone is not 

 nevertheless wholly due to the escape of adherent water, for in 

 each instance I have found more carbonic acid in the specimen 

 than was sufficient to combine with the lime to form neutral 

 carbonate ; this I have supposed to exist as bicarbonate ; but in- 

 asmuch as the carbonic acid in excess would be evolved together 

 with the loosely-combined water at the temperature of 100° C, 

 I have in every case deducted its amount from the number repre- 

 senting the total loss under those circumstances. In a similar 

 manner the entire amount of carbonic acid was deducted from 

 the total diminution of weight after ignition, in order to arrive 

 at the whole quantity of water present. The small quantity of 

 iron occurring in the three first specimens I have assumed to 

 exist as protocarbonate, although I have not determined whether 

 such is its real condition, since its amount appears to be far too 

 minute to interfere with the formula of tlie mineral ; but in 

 No. IV., in which it exists in much larger quantity, I have ascer- 

 tained that it is present entirely as sesquioxide. 



The results arrived at in the analysis of the four specimens 

 alluded to will now be given. 



2 A2 



