14 



pipes for conveying water in dwellings. The conditions under which 

 lead is dissolved by water are very complicated, and by no means 

 perfectly understood, but the following broad generalizations are justi- 

 fied by facts. Where water contains nitrates in any considerable 

 amount, and in general where water is essentially soft in character — 

 such as rain-water — the danger of lead being dissolved from the p'pe is 

 very great, and poisoning has frequentiy occurred from this source, as 

 little as one-tenth of a grain per gallon being a poisonous quantity 

 when the water is continually used, since lead is a cumulative poison. 

 Water containing less than one-fourth of this amount has been known 

 to cause serious and dangerous illness. In presence of carbonates, 

 sulphates or phosphates, a thin coating of the carb-^nate, sulphate or 

 phosphate of lead is formed on, and adheres to, the inner surface of 

 the pipe. Since these salts are practically insoluble they protect the lead 

 pipe from contact with the water and render its use quite sate. Fortu- 

 nately most natural waters contain a sufficient amount of dissolved car- 

 bonic acid or carbonates to prevent danger from the use of lead pipes 

 in their conveyance. It is. however, advisable always to alk^w water to 

 run freely for a short time where it has been stored in lead service 

 pipes over night, or for any considerable time, especially at a tempera 

 ture such as is usual in dwellings. 



The second illustration of increased solubility due to the presence 

 of a substance in solution is the case of chalk in water containing free 

 carbonic acid. The large glass vessel before you contains water par- 

 tially saturated with slaked lime. On passing carbonic acid gas from 

 the generator into this water the first effect is the conversion of some 

 of this lime into carbonate of lime or chalk ; and the great insolubility 

 of this compound causes its separation with formation of a dense white 

 precipitate which gradually settles down to ihe bottom of the vessel if 

 allowed to stand at rest. On continuing, now, to pass the carbonic 

 acid gas, after all the lime has been converted into carbonate, we 

 observe this curious effect. The liquid gradually loses its turbidity, 

 and in a few minutes is as clear and transparent as at the first. The 

 excess of carbonic acid gas has caused the precipitated chalk to 

 pass into solution. We have now what is known as hard water, 

 and its effect with soap will he apparent from the following 



