607 
of Edinburgh, Session 1881-82. 
Petrograpliie , i. 324), and described by Sterry Hunt as composed 
of a mixture of serpentine with carbonate of lime and magnesia. 
One of tliese serpentines contained 10 per cent, of carbonate of 
lime with a small percentage of carbonate of magnesia, another was 
a mixture in almost equal proportions of serpentine and dolomite. 
We had been led to believe that the specimens given us for 
analysis had been found in situ , but on inquiring particularly we 
discovered that they had been broken from some masses lying 
near a lime-kiln, and in short that they were pieces of the ordi- 
nary dolomitic limestone of the district, which had been partially 
burned in the kiln, and subsequently hydrated by exposure to the 
weather. 
We made careful analyses of the unaltered limestone with the 
view of ascertaining the precise nature of the change undergone 
by our specimens. The following numbers may be taken as 
fairly representing its average composition : — 
Si0 2 
Fe 2 0 3 ) 
A1. 2 0 3 i • 
CaO 
MgO 
C0 2 . 
9*22 
3-57 
2778 
18-25 
40-90 
9072 
Comparing this analysis with those given above of the altered 
limestone, it will be at once obvious that the difference in com- 
position is exactly of the nature which we should have expected. 
In one case the carbonic acid has been almost entirely driven off, 
and in the other case the percentage has been reduced from 40 
to 10. In other words, the silica originally present in the impure 
dolomite has, at the high temperature of the kiln, displaced the 
carbonic acid and combined with the bases. The carbonic acid 
in the altered limestone is, as already pointed out, insufficient to 
combine with all the lime, and, as we shall immediately show, 
there is good reason for believing that all the remaining carbonic 
acid is in combination with the lime, and all the magnesia and 
part of the lime in combination with the silica. If by any means 
the carbonate of lime could be separated from the silicate, the 
remainder would be a tolerably pure hydrated silicate of magnesia, 
