130 Chemistry. [Lecture 29. 



ceed till the particles are arranged at equal di- 

 stances through the whole fluid. There is, how- 

 ever, a seeming objection to this, viz. that when 

 the salt is merely thrown into the water, the par- 

 ticles do not rise and disperse themselves imme- 

 diately, but it is a long time before the salt is 

 dispersed through the whole. This, though a 

 seeming, is not a real objection. It is true, it is 

 a long time before the salt dissolves, but this is 

 in consequence of the very cause on which dis- 

 tribution depends ; and in time the salt will be 

 equally distributed through the whole. Throw 

 a heavy salt into a glass of water (sulphat of 

 copper, or blue vitriol for instance), it at first 

 sinks to the bottom, and after some days begins 

 to impart some of its colour and qualities to the 

 particles of water immediately surrounding it. 

 As the water in contact only acts upon the salt, 

 it is soon saturated, and, being thus made heavier, 

 remains round the salt in the state of an atmo- 

 sphere. The whole of the attraction of the 

 water, then, can only act on this surrounding 

 atmosphere, as chemical attraction reaches to so 

 small a distance. In a little time another stra- 

 tum will be formed, containing less salt than the 

 former. Innumerable horizontal strata will at 

 length be formed containing less salt : hence the 

 slow diffusion if not assisted with agitation. The 

 sulphuric acid is used in bleaching, by dilution 

 with the water in which the linen is steeped. 

 The bleachers at first thought it was sufficient 



