464 Messrs. J. II. and G. Gladstone on Collyrite, and a 



in the four specimens, and that composition will be that of the 

 previous specimen, minus half its silica, or SiO 2 , 4 Al 2 O 8 , 20HO. 





I. 



II. 



III. 



IV. 



Theory. 



Silicic acid 



. 7-8 



8-0 



7-8 



73 



7-7 



Alumina . 



. 51-0 



530 



51-1 



52-4 



51-5 



Water . . , 



, 41-2 



39-0 



41-1 



40-3 



40-8 



100-0 100-0 100-0 1000 100-0 



In order, however, to solve if possible the question of the 

 excess of carbonic acid, a portion of the fourth specimen was 

 finely powdered, diffused through water, and exposed to a stream 

 of carbonic acid; the gas dissolved out a little carbonate of 

 lime, which was precipitated and found to amount to 0*8 per 

 cent. It seemed incapable of dissolving out any more, leading 

 therefore to the conclusion that the amount of carbonate of 

 lime, existing as such in the mineral, is only a trace, and that 

 the remainder is in some form of combination with the alumina. 

 The powdered mineral, which had been acted on by a very large 

 amount of the gas, was afterwards analysed, and found to contain 

 very nearly all its original carbonic acid and lime. 



Extending our inquiry we attempted to form an analogous 

 compound artificially, and at once obtained a double carbonate 

 of alumina and lime, in which the carbonic acid was to the lime 

 in the ratio of three to one, and another where the ratio was 

 similar to that in the mineral. 



Another fact which bears on the state of combination of this 

 excess of carbonic acid is the following : — If the mineral, after 

 having been dried in vacuo, is exposed to a temperature of 100° C, 

 it does not lose either water or carbonic acid ; but if it be heated 

 more strongly, though not even to incipient redness, it parts 

 not only with the water and the excess of carbonic acid, but 

 also with a portion of that required to neutralize the lime, 

 and if to dull redness in a covered crucible, it parts with 

 nearly the whole of its carbonic acid ; yet a little remains which 

 cannot be driven off, even if the temperature be greatly raised. 

 Now it might be expected that simple carbonate of alumina or 

 a carbonated collyrite would be decomposed at a low heat, or 

 that bicarbonate of lime would be reduced to the common car- 

 bonate ; but the easy expulsion of the remaining carbonic acid is 

 not very compatible with either of these suppositions, and 

 appears rather to point to a double carbonate which yields up 

 all its carbonic acid more freely than carbonate of lime does. 

 The small quantity of undecomposable carbonate of lime left 

 may either have existed as such originally, or may have been 

 formed during the decomposition of the double salt. 



