of removing Matter from Solution in Water, 



29 



phragm exposed a filtering surface of about 4'38 square feet ; 

 the filtration was lateral and descending, the water entering at 

 the upper part of the right-hand compartment, and passing out 

 at the lower extremity of the left-hand division; the rate of 

 filtration being half a gallon per minute. 



A sample of the water was analysed before passing into the 

 filter, and after coming from it, after the continuance of the opera- 

 tion for a certain number of hours. The results were as follows : — 



This experiment exhibits more strikingly perhaps than any of 

 the others, the very curious circumstance that in the employ- 

 ment of charcoal the amount of impurity separated by it goes 

 on increasing to a certain point, and then diminishes ; to which, 

 however, I must again more particularly allude after describing 

 the experiment comparing sand and charcoal simultaneously ; it 

 demonstrates also very satisfactorily the power possessed by 

 charcoal, like sand, of removing not only suspended matters, but 

 also soluble salts, e. g. chloride of sodium : but to this point I 

 shall again refer. 



Although very satisfactory as showing the power of charcoal 

 under the circumstances of the experiment, yet it being pro- 

 bable that by employing the charcoal in lumps, as was done, it 

 had not a fair chance of exhibiting its peculiar properties, inas- 

 much as only very imperfect contact was efiected between the 

 impurities contained in the water and the particles of charcoal, 

 it was resolved to try an experiment with charcoal in powder, or 

 nearly so, in granules of the size of a pea. Moreover, even then 

 we should have succeeded only in proving the absolute effect of 



