7 8 Ufes of Alum in tie Arts. [Book VL 



vegetable and animal fubftances heated with alum. 

 But he did not fucceed in his attempts to form it 

 with fome other vitriolic falts. Thefe two chemifts, 

 who fuppofed alum to be a combination of the vitrio- 

 lic acid and calcareous earth, imagined that the latter 

 being converted into lime attracted the humidity of 

 the air, and produced a degree of heat fufficient to fet 

 fire to the fulphur formed by the vitriolic acid and in- 

 flammable fubftances. 



Alum is one of the moft ufeful falts in the arts. It 

 is added to tallow to make candles hard. Wood fuffi- 

 ciently foaked in alum does not eafily take fire : the 

 fame is true of paper impregnated with it, which for 

 that reafon is very proper to keep gun-powder, as it 

 alfo excludes the moifture of the air. This paper is 

 alfo ufeful in whitening filver, and in filvering brafs 

 without heat. Alum is ufeful added to milk which 

 does not eafily feparate its butter. 



It is particularly ufeful in dying, in preparing the' 

 matters to be dyed ; for by cleanfing and opening the; 

 pores upon the furface by a gentle corrofion, it both 

 renders the fubftance fit for receiving the colouring 

 particles (by which the alum is generally decompofed) 

 and at the fame time makes the colour fixed. It 

 conflitutes the bafis of crayons, which generally confift 

 of the earth of alum, finely powdered and tinged for 

 that purpofe, with different colouring matters. 



The argillaceous fofiils are referred to the following 

 heads : 



I. ARGILLA AERATA, to which the fanciful name 

 of lac lunas has been applied. This was fuppofed to 

 be a particular fpecies of calcareous earth, till M. 

 Screber found that it was a combination of argil 

 with the carbonic acid. It efiwefces with acids, 



and 



