32 CHEMICAL PHYSICS. 



the masses or quantity of matter in the two bodies, the masses must 

 be equal. That weight and mass are different ideas is evident from 

 the fact that the force with which a given body is attracted by the 

 earth varies according to latitude and elevation above the earth, while 

 the quantity of matter remains the same. But if two bodies weigh 

 the same in one place, they will do so in any other place. All our 

 weighing is a comparison with, or measurement by, some standard 

 weight, such as pound, ounce, gramme, etc. 



For scientific purposes the weight of bodies is sometimes deter- 

 mined invacuo, because it eliminates an error due to the buoyant effect 

 of atmospheric air. Weight thus determined is called absolute weighty 

 while by ordinary methods we obtain apparent weight. 



"Weights and measures. For scientific purposes the metric or decimal 

 system of weights and measures is used the world over. This system is used also 

 for general purposes by practically all except the English-speaking nations. 

 While metric weights and measures were legalized in the United States and 

 Great Britain in 1866, unfortunately neither country has as yet enforced their 

 general adoption. The U. S. Pharmacopoeia, however, uses the metric system 

 exclusively, and it finds application in all departments of the U. S. govern- 

 ment, as also for many other purposes. The basis of the metric system is a 

 quadrant (one-fourth) of the earth's circumference. This divided into ten mil- 

 lion parts gives a measure of length termed meter (39.37 inches), and this 

 unit of linear measure is the basis for the measures of extension and of weight. 



Subdivisions of all units of metric measure are denoted by prefixes of Latin 

 numerals i. e., ^ by deci, ^Q by centi, -^Q by milli ; while multiples are 

 denoted by prefixes of the Greek numerals L e., 10 by deka, 100 by hecto, 1000 

 by kilo. 



The unit of measure of capacity is the cubic decimeter called liter (1.0567 

 U. S. quart), and the unit of weight is the weight of one cubic centimeter of 

 water at the temperature of its greatest density, 4 C. (39.2 F.), and this unit 

 is called gramme (15.43-)- grains). One liter of water, equal to 1000 cubic centi- 

 meters, at its greatest density weighs 1000 grammes or one kilogram. While 

 gramme is the unit for weights up to a kilogram, the latter is the unit for all 

 larger weights, and is generally abbreviated to kilo (2.2046 pounds, avoirdupois). 



While in our country for commercial purposes the avoirdupois weight is 

 chiefly used, the apothecaries' weight is employed in this country and Great 

 Britain in prescription-writing by all who do not use the metric system. The 

 common link connecting avoirdupois, troy, apothecaries', and Imperial weights 

 is the grain, which is the same in the four systems. (For table of weights and 

 measures, see Appendix.) 



Specific weight or specific gravity denotes the weight of a body, 

 as compared with the weight of an equal bulk or equal volume of 

 another substance, which is taken as a standard or unit. This standard 

 adopted for all solids and liquids, if not otherwise stated, is water at 



