SOLUTION IN EXCESS OF PRECIPITANT. 125 



preferably filtered (Expt. 56) before adding it to the blue copper 

 test liquor. In a certain kind of disease, known as diabetes, sugar 

 is contained in the urine ; and this test is medically employed for 

 the purpose of examining the progress of the patient towards 

 recovery or otherwise, by noticing whether much or little sugar is 

 present from day to day, and regulating the treatment as this and 

 other symptoms render requisite. 



Expt. 124. Formation of a Precipitate, and Re-solution 

 thereof by means of the same Fluid. It is not unfrequently 

 observed that when a precipitate is formed by the careful addition 

 of a certain quantity of a given solution or test liquor to another 

 fluid, a further addition of the same test liquor will cause the 

 precipitate to dissolve again and form a clear fluid ; in such cases 

 the precipitate is said to be " soluble in excess " of the test liquor. 

 Several such cases have already been noticed, e.g., in Expt. 121, 

 where a scarlet precipitate, formed by adding iodide of potassium 

 to perchloride of mercury (otherwise termed corrosive sublimate), 

 was redissolved on adding excess of the first-named solution ; and 

 where the white precipitate formed by adding ammonia to zinc 

 sulphate was redissolved on adding excess of the former ; and in 

 Expt. 123, where the green precipitate formed on adding caustic 

 soda to chrome alum was soluble in excess of caustic soda. 



Take a few drops of solution of sulphate of copper in a test-tube, 

 and slowly add, drop by drop, solution of ammonia. A blue 

 precipitate will first be formed, which on further addition of 

 ammonia will be redissolved, forming a deep azure blue solution 

 very much deeper in tint than the original sulphate of copper 

 solution. This deepening in tint serves as a test for copper. 

 Thus fill a test-tube nearly full of water, and add to it one small 

 drop of sulphate of copper solution, and shake up. On looking 

 downward into the test-tube held vertically, with a piece of white 

 paper underneath, the water will appear little if anything different 

 in colour from that seen in another test-tube full of pure water 

 similarly viewed ; but on adding a few drops of ammonia solution 

 to the first test-tube and shaking up, a very distinct blue coloration 

 will now be visible on looking down the tube against the white 

 paper, and comparing the colour observed with that in the pure 

 water tube held by its side. 



Expt. 125. To detect minute Quantities of Lead or Copper 

 in Drinking Water. Sometimes natural water contains small 

 quantities of lead or copper, derived from minerals containing these 

 metals in the mountains, &c., where the water is collected from 

 the natural rainfall; more frequently pure water supplied for 

 domestic use gets contaminated with lead by passing through 



