16 PUBLIC HEALTH CHEMISTRY 



Volume and Density of Gases. All gaseous molecules, 

 at the same temperature and pressure, occupy the same 

 volume. This is another way of stating Avogadro's law, 

 that " equal volumes of all gases (at a temperature suffi- 

 ciently remote from their condensation-point the so-called 

 permanent gases) at the same temperature and pressure, 

 contain the same number of molecules." 



From this it follows, that if we know the volume occupied 

 by one gaseous molecule at standard temperature and 

 pressure, we thereby know the volume of all gaseous 

 molecules at the same temperature and pressure. The 

 volume occupied by the molecular weight of hydrogen, 

 expressed as grammes, that is 2 grammes of hydrogen gas 

 at C. and 760 mm. pressure, is 22-32 litres. Hence the 

 statement, " the molecular weight of any gas, expressed 

 in grammes, measures 22-32 litres, at standard temperature 

 and pressure, that is, C. and 760 mm., or 32 F. and 

 29-9 inches of mercury." 



The Crith. One litre of hydrogen gas at standard 

 temperature and pressure weighs 0-0896 gramme. This 

 weight has been called a crith. It follows from the above 

 statement of Avogadro's law, that one litre of oxygen gas 

 will contain the same number of molecules, each one 

 16 times the weight of the hydrogen molecule (as 32 : 2), 

 and hence the weight of one litre of oxygen gas will be 

 16 criths, or 16 X 0-0896 grm. Similarly, one litre of carbon 

 dioxide gas weighs 22 criths (as 44 : 2). The weight, there- 

 fore, of one litre of an elementary gas (with exceptions) 

 is equal to its atomic weight in criths ; and of one litre of 

 a compound gas, is equal to half its molecular weight in 

 criths. 



METRIC SYSTEM. 



The weights and measures of the metric system are 

 those used nowadays in most Public Health work, although 

 statements like " grains per gallon " still persist. The 

 chief units employed are the following : 



Length. 



i metre = the length of a rod of platinum at the temperature 

 of melting ice. This rod is kept at Paris, with official 

 copies in the large towns. Equals 39-37 inches. 



