OF THE SEVEN WELLS 



109 



on the question of filtration. The received opinion has been 

 that all impurities which are soluble cannot thus be separated > 

 and this is undoubtedly true so far as relates to the mecha- 

 nical effects obtained by the use of paper, cloths, sieves, 

 sand, &c, all of which have no chemical or special action on 

 the fluid passed through their interstices ; but if the filter be 

 made to consist partly of a porous substance such as sand, 

 and partly of another capable of exercising a chemical or 



special action, such as powdered 

 #Vide Note appended. charcoal or magnetic/* oxide of 



iron, — then the case is altered, 

 and the degree of purification to be obtained by these means 

 remains to be determined by repeated experiments on a large 

 scale. It may be said that the numerous experiments of 

 Doctor * * * and others for the various Water Companies at 

 Home have already determined this point. It appears to us, 

 notwithstanding, that this point has not been so satisfactorily 

 or clearly settled as could be wished ; inasmuch as the re- 

 sults obtained immediately after new filter beds have been 

 set up, and those obtained some time afterwards, have been 

 greatly at variance, and further, that the results obtained by 

 different experimenters have not always coincided. It is 

 evidently nothing beyond the truth for Mr. A. or Dr. B. to 

 give a certificate to the effect that a certain apparatus will 

 cause the most striking and wonderful improvement in bad 

 and foul water, &c. &c. ; but in practice and on a large scale, 

 it is not possible to renew the sand and charcoal every few 

 hours, and to have sets of sponge constantly undergoing 

 a process of purification, so as to be ready for constant 

 change. Thus it happens that what can be obtained by 

 careful operative Chemists in the Laboratory far exceeds 

 that which the practical Engineer can obtain on the large 

 scale outside, where the constant care, watching, and change 

 resorted to in the Laboratory (the re-setting up of the ap= 

 prat us as it were), is impossible, 



