-40 2 Mr* Wedgwood’s additional Obfervatlons on 
in all degrees of heat. This coincidence was not indeed effen- 
tial ; but as many degrees of heat were already before the 
public, meafured by thermometer-pieces made of the firft clay, 
and as the correfponden-ce of the firft with Fahrenheit’s 
fcale had likewife been in fome meafure afcertained, it was 
defirable that the fame degrees of heat fhould continue to be 
expreffed by the fame numbers. 
The alum earth is prepared for this purpofe by diflolving 
the alum in water, precipitating with a folution of fixed alkali, 
and wafhing the earth repeatedly with large quantities of boil- 
ing water: when the earth has fettled, the water above it is 
let off by cocks in the fides of the hogfheads ; and when the 
veflels are filled up with frefh water, care is taken to ftir up 
the earth from the bottom, and mix it thoroughly v/ith the 
liquor. I find it moft convenient to ufe the earth undried, in 
its gelatinous ftate, as in this ftate it unites eafily and perfectly 
with the clay ; whereas, when the alum earth has concreted 
into dry maffes, great labour is neceffary to mix them uni- 
formly together. 
I have tried feveral different parcels of Englifh alum, from 
the fame and from different manufadtories, and found no ma- 
terial difference in the quantity of earth it contains*. Nor 
indeed would it be of any confeqence if there was a difference 
in this refpedt, as the proportion of alum earth neceffary for 
* A difference in the quantity of earth may arife from different proportions of 
Giauber’s fait and vitriolated tartar, of which I have found quantities very 
confiderable, but nearly alike, in all the Englifh alum I have examined. Thefe 
falts are doubtlefs formed by the kelp allies employed in the preparation of the 
alum. They are difcovered by calcining the dried alum with charcoal powder, 
which decompofes the alum only, leaving the other two falts intermixed with 
the alum earth, from whence they may be extracted by water. 
different 
