272 PROFESSOR CHRISTISON ON THE ACTION OF WATER UPON LEAD. 



3. Water, which tarnishes polished lead when left at rest upon it in a glass 

 vessel for a few hours, cannot be safely transmitted through lead-pipes without 

 certain precautions.* 



4. Water which contains less than about an 8000th of salts in solution can- 

 not be safely conducted in lead-pipes, without certain precautions. 



5. Even this proportion will prove insufficient to prevent corrosion, unless a 

 considerable part of the saline matter consist of carbonates and sulphates, espe- 

 cially the former. 



6. So large a proportion as a 4000th, probably even a considerably larger 

 proportion, will be insufficient, if the salts in solution be in a great measure 

 muriates. 



7. It is, I conceive, right to add, that in all cases, even though the composi- 

 tion of the water seems to bring it within the conditions of safety now stated, an 

 attentive examination should be made of the water, after it has been running for 

 a few days through the pipes. For it is not improbable, that other circumstances, 

 besides those hitherto ascertained, may regulate the preventive influence of the 

 neutral salts. 



8. When the water is judged to be of a kind which is likely to attack lead- 

 pipes, or when it actually flows through them impregnated with lead, a remedy 

 may be found, either in leaving the pipes full of the water and at rest for three 

 or four months, or by substituting for the water a weak solution of phosphate of 

 soda in the proportion of about a 25,000th part. 



It may be mentioned, that the most convenient way to detect lead in water 

 is, first, to examine what separates on exposure to the air by dissolving it in warm 

 acetic acid, and testing the solution with sulphuretted-hydrogen, iodide of potas- 

 sium, and bichromate of potash, then, if this process fail, to concentrate the 

 water to an eighth part, and again test any insoluble matter which separates, 

 and lastly, failing this procedure also, to evaporate the water to dry ness, subject 

 the residue along with charcoal to a red-heat, act on what remains with warm 

 diluted nitric acid, and test the solution when filtered and neutralized with an 

 alkali. It may admit of question, whether, in the event of lead being indicated 

 in the last way only, the very minute quantity which may then be present can 

 prove detrimental. But this is a topic which it is foreign to my present object to 

 enter into. 



* Conversely, it is probable, though not yet proved, that, if polished lead remain untarnished or 

 nearly so for twenty-four hours in a glass of water, the water may be safely conducted through lead- 

 pipes. 



