ATTRACTION. 181 



Pneumatic Cisterns. 



Dr. Hales, more than a century ago, employed an apparatus upon 

 the principle of the modern pneumatic cistern, which was introduced 

 by Dr. Priestley. This instrument is little else than a vessel suffi- 

 ciently capacious, filled with water or quicksilver, and furnished with 

 fixed shelves and a sliding shelf. The apparatus for mercury is usu- 

 ally small, on account of the weight and expense of the metal, and 

 ounce measures are used where, in the other apparatus, we employ 

 quarts or gallons of water. In both, for the purpose of expelling the air, 

 the vessels are filled with the fluid, and then, they being inverted with 

 their mouths under it, the gas is introduced from below. The an- 

 nexed cut represents the mercurial cistern used by Dr. Hare ; it is, 

 however, five or six times larger than those generally employed. 

 This kind of cistern is rarely used, except when the gases are rapid- 

 ly absorbable by water. That in the laboratory of Yale College, is 

 of marble,* and of a similar construction, but holds not over two hun- 

 dred pounds of mercury, and usually from one hundred and fifty to- 

 one hundred and sixty pounds. 



Mercurial Cistern for gases. 

 IB - 



" B B, is a wooden box, which encloses the reservoir so as to 

 catch any of the metal which maybe spilled over the margin of the 

 cistern. This box is bottomed upon stout pieces of scantling, tenant- 

 ed together and grooved so as to conduct the mercury towards one 

 corner, where there is a spout to allow it to escape into a vessel, situ- 

 ated so as to receive it. The cistern itself, is made out of a solid 

 block of white marble. It is twenty seven inches long, twenty four 

 inches wide, and ten inches deep." 



" The ledges, S S, answer for the same purposes as the shelves in 

 the common pneumatic cistern. The excavation, w, is the well in 

 which vessels are filled with mercury, in order to be inverted and 

 placed, while full, on the ledges. There are some round holes in 



* Prof. Hitchcock, of Amherst College, has one of soap stone. 



