428 ELEMENTARY CHEMICAL MICROSCOPY 



needles or prisms of calcium sulphate results. See Calcium, 

 page 334. 



b. To the drop add a trace of Potassium Chromate, a trace of 

 Nitric Acid and a drop of Silver Nitrate. Characteristic crystals 

 of silver sulphate will be obtained, stained yellow through solid 

 solution of the silver chromate. See Silver, page 381. 



SULPHITES, THIOSULPHATES. 



a. To a drop of a solution of potassium iodate add a little 

 potato starch and a small drop of dilute sulphuric acid. Ex- 

 amine to see that no iodine has been set free. Add a fragment 

 of the unknown. The starch is colored blue. 



b. To a moderately concentrated drop of copper sulphate 

 apply a drop of a solution of the unknown by Method 777 A , 

 page 302. Warm gently — sulphites, if pure and undecom- 

 posed, yield at the most only a faint cloudiness — thiosul- 

 phates give a brown precipitate of copper sulphide and around 

 the circumference of the drop lemon-yellow crystals of copper 

 thiosulphate. 



SULPHIDES. 



a. The Silver Nitrate Precipitate was Black. 



b. Place a drop of solution or fragment of solid in the distilling 

 apparatus, cover with a slide holding a tiny drop of silver nitrate 

 and one of lead acetate side by side. Raise the cover and care- 

 fully run in a drop or two of dilute hydrochloric acid. Cover 

 quickly and allow to stand. Both drops turn black. 



c. Proceed exactly as in b but invert over the crucible a slide 

 carrying a drop of sodium nitroprusside made alkaline with 

 sodium hydroxide. A beautiful purple color results. The 

 reagent drop must be alkaline with sodium or ammonium 

 hydroxide. 



THIOCYANATES. 



a. Give a Blood-red Color with dilute Ferric Chloride. 



b. Add Mercuric Chloride and Zinc Sulphate. There will be 

 obtained the double thiocyanate of mercury and zinc. See Zinc, 



