SOME CONSEQUENCES OF GRAHAM'S WORK 605 



differences is probably to be found in the fact that, taking 

 comparable salts into account such as potassium, sodium, 

 ammonium and lithium chlorides, the less diffusive compounds 

 are the more highly hydrated. Speaking generally, the least 

 hydrated salts appear to be the most diffusive. 



But on the other hand, according to Graham, such sub- 

 stances as hydrogen cyanide, ammonia and acetic acid are less 

 diffusive than hydrogen chloride ; alcohol is less diffusive than 

 acetic acid and sulphurous acid is less diffusive than sulphuric. 

 The subject is full of riddles unsolved. 



Most interesting information of the manner in which 

 diffusion proceeds has been furnished by experiments carried 

 out by Dr. H. T. Brown in which two substances capable of 

 interacting and forming a precipitate, such as barium chloride 

 and sodium sulphate, were allowed to diffuse into one another 

 through apertures of various shapes, the one substance being 

 contained in a solid jelly, so that the form of the field of 

 diffusion became revealed by the deposition of the precipitate 

 within the jelly. When diffusion takes place through a circular 

 aperture, the precipitate takes the form of a spheroidal mass 

 resembling in shape the head of an inverted mushroom. By 

 arranging that the diffusion of the sulphate is intermittent or by 

 alternating the diffusion of a sulphate with that of a chromate, 

 well-marked zonings are produced in the diffusion spheroid which 

 indicate the zones of equal density of the diffusing substances. 



Dr. Brown has arrived at the striking conclusion that the 

 surfaces of equal density of a diffusing substance are exactly 

 analogous to the equipotential surfaces generated in an electric 

 field and that the stream lines of the diffusing substances 

 correspond to the lines or tubes of force in the electric field. 

 That this is the case is clearly shown in the accompanying 

 figures, for which I am indebted to Dr. Brown, taken from his 

 discourse published in the Proceedings of the Royal Institution, 

 1901. 



Figs. 1, 2, 3, 4 are from photographs of the diffusion spheroids produced in gelatin in 

 the manner described : i, through a semicircular aperture the straight edge of which 

 was close up to the side of the vessel containing the jelly ; 2, through a slot in a 

 plate covering the jelly ; 3, through two contiguous apertures, the diameter of which 

 was in the ratio of two to one ; and 4, from between two parallel plates. 



Figs, la, 2a, %a, 4a show the distribution of the surfaces of equal electric potential in 

 the neighbourhood of electrically charged bodies in cases corresponding to the cases 

 of diffusion that are represented. 



