4 Operations used in Quantitative Analysis [5 



which they are placed in the box or on the card. If the weight 

 on the pan be too heavy, remove it and add the next lighter 

 weight. When you have thus arrived at a weight just too 

 light, leave it on the pan and begin adding the weights 

 belonging to the next decimal place, commencing, as before, 

 with the heaviest. When all the weights on the card have 

 been tried, get the final weight accurately with the rider. 



(e) Always raise the beam from its bearings before adding 

 or removing a weight. 



(/) ^Never weigh out a substance by placing it directly on 

 the scale pan, but place it in some weighed vessel of glass, 

 porcelain, or platinum. 



(g) Never weigh substances or vessels whilst they are 

 either hotter or colder than the air inside the balance case. 



Exercise I. — Carefully clean a pair of watch glasses fitted 

 with a clip, and weigh them. Powder some oxalic acid. Place a 

 small quantity —about a gram — in the watch glasses and weigh 

 again. 



Enter the results in your note-book, thus : 



Watch glasses + clip + H 2 C 2 4 2H 2 = 



Watch glasses + clip = 



H 2 C 2 4 2H 2 

 Place the watch glasses with the acid in a desiccator (fig. 4). 



DESICCATORS 



5. The desiccator (fig. 4) is an apparatus which is 

 intended to prevent hygroscopic substances from gaining 

 weight by absorption of water. 



Substances which have been heated are apt to condense 

 moisture on their surfaces when cooling, and thus to increase 

 in weight. To prevent this a desiccator is used. The apparatus 

 shown in fig. 4 is a very convenient form. It consists of a 

 nearly air-tight vessel, the air of which is constantly kept dry 



