20 On the prefent Slate of A'irojlatton, 



The iron muft be particularly attended to to infure fuc- 

 cefs; whatever is rufly is not only not ferviceable, but actu- 

 ally injurious, by generating hydro-carbonate, a gas fpecifi- 



cally 



The quantity of hydrogen gas developed during the aftion of diluted 

 fulphuric acid upon iron, varii;^ not only accordins; to the firength of the 

 acid, but the quality of the iron employed. Dr. Prieftley (Experiments 

 and Obfervationb on Air, vol. vi ) found that annealed caft iron invariably 

 aflforded more inflammable air tiian iron which had nut undergone that 

 procefs ; the difference being nearlv i-Sth. Thus iron turnings, as being 

 in fome degree annealed, gave confidenibly more air than on Iv caft iron, 

 yet lefs than iron nails perfeftly annealed. Mr. Cavendilh alfo obtained 

 from an ounce of zinc 3 ^6 ounce meafures of inflammable air; from the 

 fame quantity of iron 412 mcahnts ; v.hile tin afiorded him only half the 

 quantity yielded by iion. Suppoimg then we are nbnut to inflate a bal- 

 loon of 30 feet dia neter, it wiil require, agreeably to Mr Cavendifti's ex- 

 periments, about two ounces and a h.ilf of iron for each cubic foot of in- 

 flammable f.ir, or iioo pounds of iron to be dilTolved in order to com- 

 pletely fill the machine ; and to produce this folution there will altogether 

 be required an equal weight of concentrated oil of vitriol, and fix times 

 this weight of water. But completely filling a balloon before its afcenfion 

 is ufeleCb ; becaufe the denfity of the furrounding medium, immediately 

 decreafing, will caufe an equally premature expanlion of the gas, and re- 

 quire its efcape even before it attains 1000 yards height : the proportional 

 ptoduft, therefore, of 2000 pounds of metal will be as much as may with 

 propriLty be applied to a machine of 30 feet diameter. 



Having proceeded according to the rules hereafter laid down for afcer- 

 taining the reqiiifite quantity of inaterials agreeably to their refpeflive 

 purity, provide next two l.-irge cylindrical wooden veflels open at the top, 

 and fhaped as inverted truncated cones, each velfel capable of containing 

 about 1000 gallons, fo that the two may hold in the fequel the whole 

 quantity of materials. In each of thefe veflels dilpol'c planes of lattice or 

 balket-woik, at intervals of fix or eight inches above each other (their 

 diameters corn ("ponding to the diameter of the part of the veflfel each 

 plane is to reft upon), adding a fufficient number- of them till within a 

 foot of the furfacc at which the diluted acid will ftand when the whole 

 quantity ihall have been added. The ule of thefe planes is for dividing 

 the iron into many portions, lb as to expofe as large a furface as polfible t» 

 the aftion of the acid : t'le weight of iron, fuppofing the veflel to contain 

 ten of the.'e planes, will be 100 pounds upon each ; which weight will 

 luffice for keeping them fteady, without having recourfe to other methods 

 of fixing them to the veflel itfclfi while the gas will eafily find a paffage 

 for itfclf from the lower through die fuperior planes by the interftices in 

 the bafkct-work. 



Having thus difpofed of the iron fo as that the two veflcls (hall contain 

 their fpecific proportion, amounting together to the whole of the required 

 quantity, provide for each vcfTcl an appropriate head, of (omcwhat lefs 

 diameter than its upper rim, th:it v hen iixed therein it may reft an inch 

 or two bt-low the level of the fame, in order that a thin ftratum of water 

 may lie over the whole furface, and render the head completely tight. 

 From the centre of the head neufr proceed a cylindricil tube of tin fix 

 inches in diameter and one foot in height ; whence forming a right angle 

 it (hould proceed about fix feet, and then dcfcend by an obtufe angle whofe 

 extremity Ihuuld terminate in an afcinding nook, converging till its ex- 

 5 treme 



