VOL. LXXIIl/j PHILOSOPHICAL TRANSACTIONS. 355 



the air rises in bubbles, as in that case it is almost impossible to shake the tube 

 soon enough, the diminution is less than it ought to be. Another great advan- 

 tage in this manner of mixing is, that the mixture receives its full diminution 

 in the short time during which it is shaken, and is not sensibly altered in bulk 

 after that ; whereas, if the airs be suffered to remain some time in contact before 

 they are shaken, they will continue diminishing for many hours. 



The reason of the abovementioned differences seems to be, that in the Abbe 

 Fon tana's method the water is shaken briskly up and down in the tube while the airs 

 are mixing, by which each small portion of the nitrous air must be in contact with 

 water, either at the instant it mixes with the common air, or at least immediately 

 after ; and it should seem, that when the airs are in contact with water during 

 the mixing, the diminution is much greater and more certain than when there is 

 no water ready to absorb the nitrous acid produced by the mixture. This induced 

 me to try whether the diminution would not be still more certain and regular if 

 one of the two kinds of air was added slowly to the other in small bubbles, while 

 the vessel containing the latter was kept continually shaking. I was not dis- 

 appointed in my expectations, as I think this method is really more accurate than 

 the Abbe Fontana's ; and besides, in the course of my experiments I had occa- 

 sion to observe a circumstance which is necessary to be attended to by those who 

 would examine the purity of air with exactness by any kind of eudiometer, as 

 well as some others which tend very much to explain many of the phenomena 

 attending the mixture of common and nitrous air. 



The apparatus I use is as follows, a (fig. 5, pi. 6), is a cylindrical glass 

 vessel, with brass caps at top and bottom : to the upper cap is fitted a brass cock b ; 

 the bottom cap is open, but is made to fit close to the brass socket Dd, and is 

 fixed in it in the same manner as a bayonet is on a musket. The socket sdhas a 

 small hole e in its bottom, and is fastened to the board of my tub by the bent 

 brass pfG, in such manner that b, the top of the cock, is about half an inch 

 under water: consequently if the vessel a is placed in its socket, with any quan- 

 tity of air in it, and the cock is then opened, the air will run out by the cock, 

 but will do so very slowly, as it can escape no faster than the water can enter by 

 the small hole e to supply its place. 



Besides this vessel, I have three glass bottles like m (fig. 6), each with a fiat 

 brass cap at bottom to make it stand steady, and a ring at top to suspend it by, 

 and also some measures of different sizes such as b (fig. 7); these are of glass 

 with a flat brass cap at bottom and a wooden handle. In using them they are 

 filled with the air wanted to be measured, and then set on the brass nob c fitted 

 on the board of my tub below the surface of the water, which drives out some 

 of the air, and leaves only the proper quantity. This measure is easier made, 

 and more expeditious in using, than the Abbe Fontana's, and I believe is equally 



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