Chap. 13.] Homier s Experiments. 77 



and this effect is not only produced by the fixed and 

 volatile alkalies, but by magnefia and calcafeous earth. 

 Alum is often obtained from the liquor of the compound 

 of iron and vitriolic acid. 



The earth of alum will combine in excefs to the fait 

 when already formed. M. Beaume boiled a folution 

 of alum with the earth precipitated from another por- 

 tion of alum, by means of fixed alkali; the earth was 

 diffolved with effervefcence, the filtrated folution had 

 no longer the tafte of alum, but that of a hard water, 

 did not redden the tincture of turnfole, but converted 

 lyrup of violets to a green. By fpontaneous evapo- 

 ration it afforded cryftals, fcaly and foft to the touch 

 like mica; M. Beaume compares them to felenite* 

 It is not eafy to compofe alum by adding vitriolic 

 acid to this faturated fait, the mixture becoming acid, 

 but not ftyptic. However, after fpontaneous eva- 

 poration for three months, the folution afforded cryi*- 

 tals of alum mixed with fome micaceous fcales, 

 fimilar to thofe afforded by alum faturated with its 

 earth. 



Alum, heated with combuflible matters, forms a 

 fubftance which takes fire on expofure to air, and is 

 called the pyrophorus of Homberg. This chemift, 

 who publifhed an account of the pyrophorus, in the 

 year 1711, made experiments on human excrement, 

 for the purpofe of obtaining a colourlefs oil poffeffing 

 the property of fixing mercury into fine filver: the 

 inquiry produced many difcoveriesj the refidue of 

 this animal fubftance, diftilled with alum, took fire on 

 expofure to the air. Homberg repeated this experi- 

 ment a number of dines, and always with fuccefs. 

 Lemery the younger, in the years 1714 and 1715* 

 publiflied two memoirs, in which he affirms, that 

 pyrophorus may be made with a great number of 



vegetable 



