8 



The first is not very exact but is handy for coarse weighing. In 

 weighing small amounts delicate instruments should be used. A 

 few siiaple rules should always be followed. 



1 . Balance pans before starting weighing. 



2. Drugs should not be placed directly on scale pans. These 

 should first be covered with pieces of paper. The opposite pan 

 should be balanced by a piece of paper of equal weight. Un- 

 less the pans are equally balanced in this manner, serious mistakes 

 may occur in weighing small amounts. The object of the paper on 

 the pans is two fold. It keeps the pan clean and insures freedom 

 from contamination from some previously weighed drug. 



3. Balance accurately with materiail t;o be weighed. When a 

 pointer is provided on the balance, it should swing an equal dis- 

 tance each side of the center or zero. 



Always throw balance off center when through weighing. This 

 stops the movements of the balance and consequently stops , its 

 wear. 



5. In weighing liquids, tare (weigh) or balance the container 

 first. 



Measuring 



This is done in graduated vessels (graduates) graduated or 

 measuring flasks and pipettes. The wider the vessel at the place of 

 reading the greater is the liability to error. On this account 

 greater accuracy can be obtained if the vessel is as narrow as pos- 

 sible where the reading is taken. The minim graduate is not ac- 

 curate for measuring small amounts, as a considerable amount of 

 the measured liquid is retained in the vessel by capillarity. A 

 minim pipette is to be preferred for this purpose and also for the 

 fact that there is less error in pouring more or less than the re- 

 quired amount into the graduate. 



The cylindrical graduate has the advantage that equal ac- 

 curacy can be obtained throughout while the conical shaped gradu- 

 ate has the advantage of greater accuracy for small amounts. A 

 few rules for measuring should always be followed. 



1. Hold the graduate so that the top 'of the liquid is a hori- 

 zontal plane perpendicular to the long axis of the graduate and have, 

 the top of the liquid on a level with the eye. 



