376 ANALYTICAL CHEMISTRY. 



notes taken during the examination, none of them should be contra- 

 dictory to the conclusions drawn. If, for instance, the preliminary 

 examination showed the substance to have been volatilized by heating 

 upon platinum foil with the exception of a very slight residue, and if, 

 afterward, other tests show the presence of ammonia and hydro- 

 chloric acid and the absence of everything else, and if, then, the con- 

 clusion be drawn that the substance is pure ammonium chloride, this 

 conclusion must be incorrect, 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 analyz- 

 ing 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 0.2-1.0 gramme. A smaller amount in fact, as little 

 as a few milligrams frequently may answer, and a much larger 

 quantity may occasionally be needed, as, for instance, in cases where 

 highly diluted reagents, such as calcium sulphate solution, lime- 

 water, hydrogen sulphide 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 possible 

 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 

 normal salts, as, for instance, aluminum sulphate, ferrous sulphate, 

 etc., have an acid reaction to litmus-paper, and that such a reaction 

 consequently is 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 



