7 6 Compofitlon of Jlum. [Book VI. 



very hot one, and it was made of coal cinders j the 

 faucer was placed on the bar of the grate, and the 

 alum, he conceives, was formed from the fulphureous 

 acid of the cinders uniting itfelf with the clay, which 

 enters into the compofition of the yellow ware. He 

 was informed that the vapour which, in fome places, 

 efcapes from the coal-pits which are on fire in Staf- 

 fordfhire, forms an alum whenever it meets with an ar- 

 gillaceous earth. This, he remarks, is conformable to 

 the manner of making alum on the Solfatara, near 

 Naples, where they place little heaps of argillaceous 

 earths or ftones over the crevices from which the ful- 

 phureous vapour iffues, in order that they may colled 

 a greater quantity of alum. 



This fait contains much water, hence it undergoes 

 what chemifts call the watry fufion; after this there 

 remains a fubftance called burnt alum, which contains 

 the earth and vitriolic acid, with little or no water ; 

 this, if the heat is increafed, does not melt, nor by heat 

 alone can we feparate the whole of the acid, though 

 fome of it rifes j but the addition of any inflammable 

 matter difpofes it to rift; in fumes, which are very ful- 

 phureous. One hundred parts of cryftallized alum 

 contain thirty-eight of vitriolic acid, eighteen of clay> 

 and forty-four of water. 



The fenfible qualities of alum mew it to be the op- 

 pofite to borax, which is compofed of the fixed alkali 

 and a very weak acid ; on the contrary, alum is the 

 ilrongeft acid, combined with an earth which attracts 

 it very weakly ; fo that the qualities of the acid are 

 very little altered, for if we apply a fohition of alum 

 to the infufion of litmus, it changes it to a red. The 

 folution has alfo a manifeft acidity, combined with; 

 another tafte which approaches to fwetrtnefs. If an 

 alkali is added, the earth is immediately precipitated, 



