De/crlption of a Mercurial Air-holder. 149^ 



cur)', it is balanced by the fmall weight K, which hangs 

 from that part of the fpiral where the radius is equal to thai 

 of the wheel G ; from this point the radius of the fpiral mud 

 be increafed in fuih proportion, that in every part of its cir- 

 cuit, the weight K may be an cxaft counterpoife to the air- 

 holder. In this way, fo little friftion will be produced, that 

 merely pluaging the lower oritice of the tube D under mer- 

 cury contained in the fmall veflel L, will be fufficient to 

 overcome every refiftance, and to force the gas difcharged 

 from the beak of a retort into the receiver, v^^here, whatever 

 may be its quantity, it will be fubjetled to a prefllire exaftly 

 correfponding to that of the atmofphere. The edge of the 

 wheel H being graduated, the balance-cord c may be made 

 to indicate its volume. 



Should it at any time be neceffary to reduce the prefllire 

 to the medium ftandard of the barometer, it may eafily be 

 done by graduating the lower end of the tube D, and adding 

 to the weights I or K, as may be found neceffary' ; the fur- 

 face of the mercury in the tube pointing out the increafe or 

 diminution. 



The concavity at the top of the internal cylinder is intended 

 to contain any liquid it may be thought proper to expofe to 

 the action of the gas. 



The upper orificey) with its ground- ftopper, is particularlv 

 ufeful in conveying air from the retort g, with its curved 

 neck, into the receiver, without its pafling through tha 

 tube D. In all cafes where a rapid extrication of gas is ex- 

 pefted, the retort g Ibould be firmly luted to the orifice, and 

 the weight I removed from the top of the receiver; this, bv 

 diminifhing the preffure, will admit the gas to expand freely in 

 the air-holder at the inltant of its formation, and prevent an 

 •xplofioii of the veflels. The fame caution mull be obferved 

 whenever any inflammation of gas is produced by the elcclric 

 fpark. 



The air may be readily transferred through water, or even 

 mercury, by the tube h. Fig. i. 



To prevent an abforplion of mercury In cafe of a condenfa- 



tion taking place in the retort made ufe of for generating air, 



Mr. Davy has applied the ftop-cock i, to which the neck is 



b firmly 



