Colouring Malkr of Lapis Lazuli, iifc. 313 



compound. A very ftrong proof to the contracy offered itfelf In one of the fynthetical 

 operations. Sulphate of lime abounding with iron having been treated with powdered 

 charcoal, and then kept in digeftion in nitrous acid ; the pruffiate of pot-alh at firft had 

 no other efFeft but to turn the fluid green, without affording any precipitate j the addition 

 of a folution of alumine immediately determined a green precipitate, which was taken up 

 by acids, and was a£l:ed upon in all refpe£ls like that which was obtained from the decom- 

 pofuion of the lapis. 



CONCLUSION. 



I return to the confequences which appear to me to refult from the fa£ls laid down in 

 this memoir. 



1°. The fulphatc of lime of Montolier is coloured by a red oxide of iron, which adheres 

 fo flrongly to the filiceous earth as to refift the aftion of acids. 



2°. This fulphate, treated with charcoal, produces a fulphuret of iron, in which this 

 metal is lefs oxided, which being difTolved in acids, no longer affords a PrufTian blue by 

 the pruffiates, but a green precipitate, which is deflroyed, inflead of being brightened, by 

 acids, and which preferves its peculiar blue colour in pot-afh itfelf, and in the fire which 

 its dry fufion requires. 



3°. In operating upon fulphuret of iron prepared In the dire£l way, a produfk is ob- 

 tained which manifefts the fame properties in the fame acids, and by the fame re-agents. 



4°. Thefe phenomena are exa£lly fimilar to thofe which the lapis prefents when it is 

 fubjefled to the fame operations. 



5°. So that we can at pleafure form the blue colouring principle of the lapis, with the 

 only difference which neceffarily refults in the natural produ£l, from the flow combination 

 of the principle with the earths and the fulphate of lime. 



6°. In a word, the blue fulphuret of iron is the true and only colouring principle of 

 all the varieties of the lapis, and probably alfo of the mineral known by the name of 

 blue ftone of Vorau. 



Experiments made with the Metallic Pile of Signor Volla, principally direBed to afcertain 



the Powers of different Metallic Bodies. By Ueut-. Col. Henry Haldane. 



LETTER III. 



To Mr. NICHOLSON. 

 S I R, 



T 



JL HE apparatus I have ufed in the following experiments Is of averyfimplc conftru£lion. 



A deal board, of about fix feet long, three inches wide, and one inch in thicknefs. The 



middle of the board is fluted longitudinally, the channel being 0.6 inch wide. The 



Vol. IV. — October 1800. S s metallic 



