60 PRESCRIPTION WRITING 



^ riuidextracti Nucis Vomica 5 ij 



Liquoris Potassii Arsenitis S viij 



the total amount of mixture would be 10 ounces. This could not 

 be easily divided into sixteen doses, and the dose would not conform 

 to the usual measures, and furthermore would require careful calcu- 

 lation. 



Prescriptions containing solids and liquids show this necessity 

 more prominently than those of liquids alone, because we cannot tell 

 just how much volume the dissolved solid will have. 



Furthermore, such quantities make irregular amoimts and do not 

 correspond to usual and standard bottles. 



In figuring the quantities, although domestic measures vary, we 

 may assume that an ounce bottle will hold eight teaspoonful doses, or 

 two tablespoonful doses, a two ounce bottle, fifteen or sixteen tea- 

 spoonful doses, a four ounce bottle, 30 teaspoonfuls or eight table- 

 spoonfuls, and an eight ounce bottle, 15-16 tablespoonfuls, etc. 



PERCENTAGE SOLUTIONS 



Although making percentage solutions is a simple matter of arith- 

 metic, the siibject is difficult for many to grasp. There are various 

 ways of figuring the correct amount, some of which may be illustrated 

 as follows : 



Figuring the amount for one ounce and multiplying by the total 

 number of ounces. Thus : 



1 ounce=8 drams^480 minims. 1 per cent, of 480 minims is 

 4.8 minims; for ordinary purposes, 5 minims. In other words, 

 there are 4.8 minims of substance in one ounce of a 1 per cent, 

 solution. When this amount is obtained any amount may be easily 

 figured for stronger solutions or for larger quantities. 



After a little experience, a prescriber will remember that 4.8 

 minims (for ordinary purposes 5 minims) are required to make one 

 ounce of a 1 per cent, solution. Then deductions are easily made for 

 other strength solutions and for various amounts. 



In making absolutely correct percentage solutions all items must 

 be weighed or measured, not weighed and measured in the same 

 preparation. The weight of 480 minims of water under stand- 

 ard conditions is 454.6 grains. Therefore 4.8 grains of sub- 

 stance with enough water to make an ounce is not absolutely a 

 1 per cent, solution by weight or volume, but such exactness is 

 rarely necessary, and it is sufficient to carry in mind that 4.5 grains 

 (4.546 grains), practically 5 grains, is the quantity of substance 

 required to make 1 fluid ounce of a 1 per cent, aqueous solution of 

 solids. For ordinary purposes it is sufficient to calculate on the basis 

 of 500 minims or grains to the ounce. 



Percentage Solutions m the Metric System. Since the 



