981 



Hydrogen Acids, — ^The period of diffusion arbitrarily chosen for 

 these acids was five days. The diffusate, or quantity of acid dif- 

 fused, was determined by precipitating the liquid of the external 

 reservoirs with nitrate of silver, and weighing the salt of silver 

 formed. 



Diffusion of hydrochloric acid in five days, at 51° F., in two 



cells : — 



Grs. Ratio. 



From I per cent, solution 7*41 0*97 



From 2 per cent, solution 15*04? 2*00 



From 4 per cent, solution 30*72 -l-'OS 



From 8 per cent, solution 67*68 9-00 



Hydrochloric acid is the most highly diffusive substance hitherto 

 observed ; it appears to exceed hydrate of potash at 53°*5, as 7*56 

 to 6-12, or as 100 to 80-9. 



The experiments indicate a similarity of diffusion between the 

 isomorphous substances, hydrochloric and hydriodic acids, and hy- 

 drochloric and hydrobromic acids. 



Diffusate from 2 per cent, solutions at 51° F. : — 



Hydrochloric acid 15*04 100 



Hydriodic acid 15*11 100*46 



Diff*usate from 2 per cent, solutions at 59°*7 F. : — 



Hydrochloric acid 16*55 100 



Hydrobromic acid 16*58 100*18 



Hydrobromic acid appears therefore to coincide in diffusibility 

 with hydrochloric acid at this temperature. It is remarked that 

 these three acids, hydrochloric, hydrobromic and hydriodic, do not 

 exhibit the same correspondence in another physical property, 

 namely, the densities of their aqueous solutions containing the same 

 proportion of acid. The densities of 2 per cent, solutions of hydro- 

 chloric and hydriodic acids appear to be respectively 1*0104 and 

 1*0143, at 60° F., and that of hydrobromic acid is an intermediate 

 number. The same acids are also known to differ considerably in 

 the boiling-points of solutions containing the same proportion of 

 acid. A considerable diversity of physical properties appears here 

 to be compatible with equal diff*usibility in substances which are 

 isomorphous. 



The difi'usion-time of bromine was made ten days, or double the 

 time of hydrobromic acid. Two cells contained together a diffusate 

 of 5*80 grs. of bromine; another two cells a diffusate of 5*88 grs.; 

 mean 5*84 grs. at 60°*1 F. ; or 6*76 grs. for a 1 per cent, solution. 

 Doubling the last result, 13*52 grs. are obtained for a 2 per cent, 

 solution, which is still considerably under the diffusate of hydro- 

 bromic acid (16*58 grs.) in half the time. 



Hydrocyanic acid appeared less diffusive than hydrochloric acid, 

 at the same temperature 59°*7, as 12*45 to 16*55, or as 75*2 to 100, 

 and not to belong therefore to the same class of diffusive substances. 



Nitric Acid.*— Time, of diffusion also five days. The quantity of 



