1 6 Operations used in Quantitative Analysis [17, 18 



meshes, and the cover placed loosely on the tin, which is heated 

 by means of a Bunsen burner. 



IGNITION OF PRECIPITATES 



17. For this operation vessels made of porcelain, nickel, and 

 platinum are used. Of these the platinum vessel is the most 

 convenient, but unfortunately it tends to form fusible alloys 

 with the heavy metals. The agricultural analyst, however, has 

 so very seldom to ignite precipitates which are hurtful to 



Fig. 16. — Argands. 



Fig. 17.— Fletcher. 



platinum that he may use vessels of that metal almost exclu- 

 sively. The most useful form is a capsule 1 inch deep by 

 2 inches diameter, as shown in fig. 16. 



18. When a platinum vessel is frequently heated over a 

 Bunsen flame it tends to blister, and a grey deposit forms 

 where the flame has touched it. After a little while it begins 

 to lose weight, and in time the capsule becomes worn out. 



This may be avoided in a great measure by cleaning the 

 vessel frequently with sea-sand, whose rounded granules 

 smooth down the minute blisters which give the grey ap- 

 pearance. 



