322 HOW TO SEE WITH THE MICROSCOPE. 



by the profession, and confine himself to that one simple 

 unit the grain. This, in the case of chemicals, can be 

 adopted with perfect ease, and much confusion thus 

 avoided. In the following pages no other weight will 

 be mentioned. Referring to liquid measures, the case 

 would be somewhat modified, and we shall have occasion 

 probably to speak of fluid ounces, drachms, etc. Now 

 there are two fluid ounces in vogue; the one is known 

 as the English " imperial," and is equivalent to a volume 

 of distilled water at 60 F., weighing 437.5 grains. 

 This, also, is the ounce avoirdupois. The United States 

 Standard fluid ounce is quite another thing. This is equal 

 to a volume of distilled water at 60 F., weighing 455.69 

 grains, sixteen such ounces making one pint. The United 

 States ounce will be the one referred to in this book, 

 when speaking of fluid ounces and drachms, eight of the 

 latter being equal to one of the former. The practi- 

 tioner should keep in mind these differences between the 

 United States and the English fluid ounce, when reading 

 English authors. 



In the selection of his chemicals for the preparation 

 of the standard solutions, the utmost attention must be 

 paid to their purity. They should not be bought at the 

 "nearest drug store." It will be always advisable to 

 purchase them from leading dealers. The formulas 

 which will be given relate only to pure chemicals. Due 

 attention also should be given to the manipulations, 

 that the chemicals shall not become contaminated. Even 

 after the standard solutions are successfully prepared, 

 the careless operator may ruin a solution by thought- 



