142 PHILOSOPHICAL TRANSACTIONS. [anNO 1786. 



natural substance possessed of that property, which would not at the same time 

 render the compound too vitrescible, I was obliged to have recourse to some 

 artificial preparation ; and as the earth of alum is the pure argillaceous earth, to 

 which all clays owe their property of diminution in the fire, possessing that pro- 

 perty in a greater or less degree according to the quantity of alum earth in their 

 composition, I mixed some of this earth with the clay, and found it to answer 

 my wishes completely, both in procuring the necessary degree of diminution, 

 and increasing its unvitrescibility. So little is this compound disposed to vitri- 

 fication, that the greatest heat I could give it, that of l6o°, did not even bring 

 it to a porcelain texture, but left it still bibulous ; and as it does not arrive at the 

 porcelain state in this fire, there can be no danger of its approaching too near 

 to the vitrescent in any heat that we can produce in a furnace. 



In order to obtain the exact medium required, I took one of the best of the 

 clays I had procured from Cornwall, and mixed it with different proportions of 

 the alum earth, till the composition was found, on repeated trials, to agree with 

 the original in all degrees of heat. This coincidence was not indeed essential ; 

 but as many degrees of heat were already before the public, measured by ther- 

 mometer pieces made of the first clay, and as the correspondence of the first 

 with Fahrenheit's scale had likewise been in some measure ascertained, it was 

 desirable that the same degrees of heat should continue to be expressed by the 

 same numbers. 



The alum earth is prepared for this purpose by dissolving the alum in water, 

 precipitating with a solution of fixed alkali, and washing the earth repeatedly 

 with large quantities of boiling vvater ; when the earth has settled, the water 

 above it is let off by cocks in the sides of the hogsheads ; and when the vessels 

 are filled up with fresh water, care is taken to stir up the earth from the bottom, 

 and mix it thoroughly with the liquor. I find it most convenient to use the 

 earth undried, in its gelatinous state, as in this state it unites easily and per- 

 fectly with the clay ; whereas, when the alum earth has concreted into dry 

 masses, great labour is necessary to mix them uniformly together. 



I have tried several different parcels of English alum, from the same and from 

 different manufactories, and found no material difference in the quantity of earth 

 it contains. Nor indeed would it be of any consequence if there was a differ- 

 ence in this respect, as the proportion of alum earth necessary for different clays, 

 and even'for different parcels of the same clay, can only be ascertained by re- 

 peated trials, adding successive quantities of the earth till the desired effect is 

 found to be produced. Ten hundred weight of the Cornwall porcelain clay, 

 which I have now in use, required all the earth that was afforded by five hun- 

 dred weight of alum. 



It is material in this place to observe, that the earth of alum is extremely 



