CHEMICAL UNITS OF WEIGHT. FORMULAE 75 







rule to and 760 mm., and the weight of 22.4 liters is calculated 

 from the reduced volume and the measured weight. 



These standard or unit weights of substances are commonly 

 called molecular weights (see p. 83). 



Unit or Standard Weights of the Elements. Let us now 



examine the weights of the constituent elements making up a 

 cubeful of each substance, as shown in the last three columns of 

 the above table. We must first be sure we understand what these 

 numbers are. They are combining proportions, such as we have 

 given on previous occasions (e.g., pp. 15, 20, 31). They are equiv- 

 alents (p. 53). We can use them in our condensed form for 

 representing chemical changes: 



Oxygen (16) + Hydrogen (2.016) -> Water (18.016). 



Hydrogen (1.008) + Chlorine (35.46) -> Hydrogen chloride 



(36.468). 



We observe at once that the weights in the oxygen column, or 

 the chlorine column, for example, are not identical. There was 

 no reason to expect that they would be alike, since different sub- 

 stances differ in composition. But we do observe that the weights 

 of any one element are all exact multiples of the smallest number in its 

 column, either by unity or some other whole number. Thus, for 

 hydrogen, the weights are: 2.016, 2.016 and 1.008. The unit 

 weight of water contains exactly the same weight of hydrogen as 

 does the unit weight of hydrogen itself, and exactly twice as much 

 as does the unit weight of hydrogen chloride. Similar relations 

 hold in the oxygen and chlorine columns. This is a very surpris- 

 ing, natural fact, and, better still, one for which we instantly per- 

 ceive a use. This fact greatly simplifies our task of finding some 

 way of expressing the compositions of substances in a simple 

 manner. The fact does not apply to a few compounds only. If 

 our table had included all the hundreds of compounds of chlorine 

 (for example) which are capable of being converted into vapor, we 

 should have found, indeed, many multiples of 35.46 larger than 

 the two units (70.92) in chlorine monoxide, but no number smaller 



