Prof. Graham on the Law of the Diffusion of Gases. 187 



air-pump in perfect order, and exhausted. When the stop- 

 cock of the receiver was closed, nothing entered the exhaust- 

 ed receiver; but on opening it, either air entered, forcing its 

 way through the pores of the stucco, or any gas which might 

 be conducted to it, by means of a flexible tube from a proper 

 magazine. 



The time was noted in which the mercury of the gauge- 

 barometer, in communication with the receiver, fell two inches, 

 always setting out with gas of the tension of one inch mercury 

 in the receiver, and stopping exactly when it attained a ten- 

 sion of three inches. 



Air entered, according to eight or ten experiments made 

 on different days, in within ten seconds, more or less, of ten 

 minutes, and so whether the air was saturated with aqueous 

 vapour or dry. 



The same volume of different gases entered in the times 

 expressed in the following table, under the same pressure, or 

 beginning at a pressure of 29 inches mercury, and terminating 

 with a pressure of 27 inches: 



Min. Sec. 



Air, dry 10 



Air, saturated with moisture at 60° ... 10 



Carbonic acid 10 



Nitrogen 10 



Oxygen 10 O'^T 



Carbonic oxide 9 30 



defiant gas _ 7 50 



Coal gas. 7 



Hydrogen 4 



In repetitions of the experiments, the numbers oscillated 

 10, or 12, sometimes 20 seconds, on either side of the numbers 

 given in the table, from circumstances which could not easily 

 be appreciated. As the mercury in the gauge fell not conti- 

 nuously, but by leaps, from adhesion to the glass, the experi- 

 ments are not susceptible of the greatest accuracy. 



The greater the pressure the more rapidly are gases forced 

 through the pores of the plug; but the quantity of gas which 

 penetrates in any given time is not exactly proportional to the 

 pressure, at least in the case of air and hydrogen. By dou- 

 bling the pressure, we do not quite so much as double the 

 quantity of gas forced through ; or a fixed quantity of gas 

 does not enter in half time under double pressure, as will be 

 evident from the following table of observations. Pressure of 

 atmosphere 30 inches. 



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