204 ANALYTICAL CHEMISTRY. 



stance is pure ammonium chloride, this conclusion must be in- 

 correct, because pure ammonium chloride is wholly volatile and 

 does not leave a residue. It will then be the task of the operator 

 to find where the mistake occurred, and to correct it. 



Use of reagents. A mistake made by most beginners in 

 analyzing is the use of too large quantities both of the substance 

 applied for testing and of the reagents added. This excessive use 

 of material is not only a waste of money, but, what is of greater 

 importance, a waste of time. Some experience in analyzing 

 will soon convince the student of the truth contained in this 

 remark, and will also enable him to select the correct quantities 

 of materials to be used, which rarely exceed 0.5-1 gram. A 

 smaller amount may frequently answer and a much larger quan- 

 tity may occasionally be needed, as, for instance, in such cases 

 where highly diluted reagents, such as calcium sulphate solution, 

 lime-water, hydrosulphuric acid-water, etc.. are applied. 



Preliminary examination. This examination includes the fol- 

 lowing points : 



1. Physical properties. Solid or liquid; crystallized or amor- 

 phous ; color, odor, hardness, gravity, etc. (On account of pos- 

 sible poisonous properties, the greatest care should be exercised 

 in tasting a substance.) 



2. Action on litmus. Examined by holding litmus-paper in 

 the liquid, or by placing the powdered solid upon red and blue 

 litmus-paper, moistened with water. (It should be remembered 

 that many neutral salts, as, for instance, aluminium sulphate, 

 ferrous sulphate, etc., have an acid reaction to litmus-paper, 

 and that such a reaction is consequently not conclusive of the 

 presence of a free acid, nor even of an acid salt.) 



3. Heating on platinum foil or in a dry glass tube, open at both ends. 

 (If the substance to be examined be a liquid, it should be evap- 

 orated in a small porcelain dish to see whether a solid residue 

 be left or not. If a residue be left, it should be treated like 

 a solid.) The heating of a small quantity of a solid sub- 

 stance upon platinum foil held over the flame of a Bunsen- 





