Prof. Graham on the Law of the Diffusion of Gases. 353 



simply determined by filling it with water, and weighing the 

 water which it contains, and which can be poured from it into 

 a counterpoised phial. Then, after any experiment, the re- 

 turn-air may be found from the weight of the water which has 

 entered the instrument, determined in the same manner. By 

 proceeding in this way, we avoid wetting the stucco after every 

 experiment. A hood of damp paper may be inverted over 

 the upper tube while the diffusion is going on, and the whole 

 counterpoised in a tumbler of water, being suspended from 

 one of the arms of the beam of a balance, the scale on that side 

 being removed. An experiment with the bulb will generally 

 occupy several hours. But with a plain diffusion-tube, a much 

 shorter time will suffice. 



A peculiar advantage of this mode of taking the specific 

 gravity of gases, besides its simplicity, is, that we can operate 

 upon a most minute quantity of gas : it is possible to come 

 within lOOdth of the specific gravity, operating upon no more 

 than one cubic inch of gas. 



It is to be regretted that this method is not so fully avail- 

 able in the case of coal-gas, as might be expected. The den- 

 sity of that gaseous mixture appears to depend, in no incon- 

 siderable measure, upon the presence of a small quantity of 

 the heavier hydro-carburets, such as naphtha-vapour ; and 

 these are apt to be absorbed and withdrawn in part by the 

 water, during the continuance of a diffusion experiment. I 

 have observed coal-gas to contract T ' n th of its bulk by stand- 

 ing over water, without agitation, for forty-eight hours, and 

 from the loss of the denser portion of it. But in the case of 

 this gas, the experiment should succeed over brine, which ab- 

 sorbs much less of the gas than water does. 



The process of diffusion may be managed so as to demon- 

 strate relations in density. The short upper tubes of two dif- 

 fusion-bulbs, not closed by plaster, but open, were connected 

 by means of thick caoutchouc adopters, with the two ends of 

 a short piece of straight tube, in which there was a diaphragm 

 of plaster, J-th of an inch in thickness, and equidistant from 

 either end of the tube. The apparatus being proved air-tight, 

 and the plug in a proper condition for diffusion, one of the 

 diffusion-bulbs was filled with nitrogen gas,"and the other with 

 carbonic oxide, and the bulbs placed upright in separate con- 

 tiguous glasses containing water. The quantity of gas in each 

 was carefully observed at the beginning of the experiment, 

 and after the expiry of twenty-four hours, when it was found 

 to be identically the same as at first; at least, if a contraction 

 or expansion took place, it was the same in both bulbs, and 

 therefore entirely due to changes in temperature or pressure. 

 Third Series. Vol.2. No. 11. May 1833. 2 Z 



