Mitchell on the Penetrativeness of Fluids. 105 



rial trough, was placed the vessel containing the gas to be tried, 

 and its velocity of penetration measured by the time occupied in 

 elevating to a given degree the mercurial column in the other limb. 

 Having thus compared the gases with common air, and subse- 

 quently by the same instrument, arid in bottles, with each other, I 

 was able to arrange the following gases according to their relative 

 facility of transmission, beginning with the most powerful : am- 

 monia, sulphuretted hydrogen, cyanogen, carbonic acid, nitrous 

 oxide, arsenuretted hydrogen, olefiant gas, hydrogen, oxygen, car- 

 bonic oxide, and nitrogen. 



Ammonia transmitted in 1 minute as much in volume as sulphu- 

 retted hydrogen in 2J minutes cyanogen, 3j carbonic acid, 5J 

 nitrous oxide, 6J arsenuretted hydrogen, 27j olefiant gas, 28 

 hydrogen 37 J oxygen, 1 hour and 53 minutes carbonic oxide, 2 

 hours and 40 minutes. 



Nitrogen has a rate of penetration so low as to be difficult to 

 ascertain, because there is no gas of a lower rate with which to com- 

 pare it. Only by causing it to pass through a membrane by means 

 of a column of mercury, is the fact of its transmission known. In 

 that way, the quantity being compared with that of carbonic acid, its 

 rate was found to be about three hours and a quarter*. This ex- 

 periment, made but once, is not confidently relied on ; but the rate 

 of nitrogen is unquestionably less than that of carbonic oxide. 



Chlorine immediately altered the texture of the membrane, as did 

 muriatic acid gas, sulphurous acid, nitric oxide, and some others, so 

 that it was impossible to reach, for their rate of penetration, accu- 

 rate results. 



In every case the movement of the gas through the membrane 

 became progressively slower, until it totally ceased ; and finally, but 

 more slowly, the mixed gas returned, as indicated by the descent of 

 the column of mercury. The retrogradation ceased only when the 

 two columns came to equilibrium, or, failing the possibility of that, 

 when the mercury in the shorter limb had reached the membrane, 

 through which mercury has not been found able to penetrate. 



Acquainted with the fact, and the relative rate of the penetrative- 

 ness of gases, the degree of force became the next subject of inquiry : 



* A vessel filled with atmospheric air and closed by gum elastic was submerged 

 under water for two weeks, when it was found to contain only nitrogen gas. Pos- 

 sibly this arrangement may furnish a new eudiometer. It offers a new mode 

 of obtaining nitrogen gas. 



A phial containing atmospheric air, after being closed by a membrane, was 

 placed in a receiver holding nitrous oxide. In about two weeks only nitrogen 

 was found in the phial. These facts show the mechanically sluggish character of 

 nitrogen gas : with its chemical inactivity we have been long acquainted. 



