DETECTION OF POISONOUS METALS. 127 



filtered fluid will turn black with sulphuretted hydrogen solution 

 if copper be present ; whilst other tests for copper are described 

 in Expts. 95 and 124. 



When copper or brass cooking utensils are 

 employed in the kitchen, it often happens that 

 these impregnate the food cooked therein with 

 copper, more especially when the vessels are 

 not scrupulously cleansed before use. Acid 

 fluids and substances containing sugar (also 

 oily matters, such as butter, &c.), if left in 

 contact with copper and air for some time, will * Jj, 5 '. Pipe-Clay 

 often cause a little copper to become oxidised 

 and dissolved, much as ammonia does under similar conditions 

 (Expt. 114). Hence such vessels ought to be well tinned inside, 

 so as to prevent the copper or brass from coming into direct con- 

 tact with the food. In case it is desired to test food to see if 

 copper has been thus taken up, the method employed is usually 

 that above described, viz., drying and burning to ashes (or incin- 

 erating) in a large platinum dish, and then testing the ashes. 



A similar process (with appropriate modifications as to the final 

 tests) is used for the detection of lead. One of the best tests in 

 this case is to evaporate a little of the nitric acid solution of the 

 ashes to dryness over a steam bath (Expt. 89), so as to get rid of 

 any excess of nitric acid ; the residue is dissolved in a few drops 

 of water, and a drop of solution of potassium chromate added, 

 when a yellow precipitate is formed of chromate of lead,* if lead 

 is present to any considerable extent; whilst more minute 

 quantities still can be found by means of sulphuretted hydrogen 

 water. Lead is often contained in tinned vegetables (especially 

 where sugar, acids, or sweet juice is largely present, as with canned 

 peaches and apricots, pine apples in slices, &c.), being derived 

 from impurities contained in the tin, and from the solderings of 

 the cans. 



Expt. 127. To pour out apparently Blue Ink, Brown Sherry, 

 or Milk from the same Water Bottle into different Wine Glasses. 

 A curious effect may be produced by taking advantage of the 

 property possessed by ferrocyanide of potassium of forming 

 differently coloured precipitates with different metallic compounds. 

 Einse out three wine-glasses, one with a solution of perchloride 0} 

 iron, the second with a solution of sulphate of copper, and the 

 third with a solution of sulphate of zinc, so that a few drops of each 



* "Chrome yellow," used by artists, is essentially chromate of lead pre- 

 pared substantially by this process, but with various modifications for the 

 purpose of obtaining different shades. 



