^24 Themmena obj'aved in the Air-VuuU 



5. We may therefore form at pleafure the blue colouring 

 principle of lapis lazuli, the difference in the natural pro- 

 du6lion neceflarily refulting from the flow, combination of 

 this principle with the earths and fulphat of lime alone ex- 

 cepted. 



6. In the laft place, the blue fuifure of iron Is the real and 

 fole colouring principle of all the varieties of the lapis lazuli, 

 and probably of that mineral alfo known under the name of 

 the blue Itone of Vorau. 



Yl. Account of certain Phenom^n^ ohferi<e(l,in,fhe. Air-Vault 



of the Furnaces of the Devon Iron- JJforks "* ; together %inth 



fome pra6lical Reviarks o?i the ISlanagementof \^laJl-Fur- 



■nctces. By Mr. RoEBUCK, in a^Letter to. Sir James 



■ Hal.l, Bart. Coinvmnicated.hy Sir James HALLf. 



I 



HAVE examined my meinbrandums concerning the 

 obfervations T made on the condenfed air in the air- vault 

 of the Devon iron-works near Alloa ; and, according to your 

 requeft, I now tranfmit you an account of them ; and alfo of 

 an experiment I made, when a partner and manager of thefe' 

 works, in order to increase the produce of blaft-furnaces. • 



The two blaft-furnaces at Devon are of large dimenfions,' 

 each b(^ing 44 feet high, and about 13 feet wide in the 

 bofhes, or wideft part, and are formed on a fleep bank, by 

 two pits funk in a very folid ftratum of coarfe-arained free- 

 ftone. '^-V'^'"''^ •^.-. 



Thefe pits were afterwards fli aped and lined, in the ufual 

 manner of blaft-furnaces, with common bricks and fire- 

 bricks; and the hearth was laid with large blocks of the 

 ftone that had been dug out, and which ferve the purpofe of 

 fire-ftones. At the back of the two furnaces, next the bank, 

 the air-vault is excavated, and formed by a mine drove in 

 the folid rock, diftant from the furnaces about x6 feet. The 



• Thefe iron-works are on the banks of .the river Devon, which runs 

 into the Frith of Forth near Alloa. They ai'e three miles from Alloa, 

 and eitrhi from Stirling. 



f From I'rafifailions of the Royal Society of Edinburgh y Vol. V. part i. 



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