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POPULAR SCIENCE REVIEW. 
and hydrogen referred to in the preceding sentence, the re- 
sultant gas occupying 2 volumes. Hence we conclude that, so far 
as the power of combining with or of replacing hydrogen is 
concerned, the amount of carbon expressed by the combining 
weight is equivalent to twice the amount of oxygen expressed 
by the combining weight of that element, or to 1^- time the 
amount of nitrogen expressed by the combining weight of that 
element. Or, if the letters C, 0, and N represent 12, 16, and 
14 parts of the elements carbon, oxygen, and nitrogen respec- 
tively, then, so far as the power of combining with or of 
replacing hydrogen is concerned, 
C = 20 
3C = 4N ; 
or to put the same statement in another form : 
C = 4H 
0 = 2H 
N = 3H. 
In such form as this have the long known facts concern- 
ing equivalency found a place in modem chemistry. We 
compare the amounts of the elements expressed by their com- 
bining weights (deduced from the two-volume formulse of 
compounds) with reference to their power of replacing or of 
combining with hydrogen, and we arrive at certain wide and 
well-marked generalizations. 
The new chemistry finds a place for the idea of equivalency. 
If the combining number of each element be divided by 
the equivalent of that element, a number is obtained expressing 
the valency of the element. But we have found that an element 
may have more than one equivalent ; the valency of an element 
may therefore vary. Each element has, however, a maximum 
valency.* 
The combining numbers of the elements are deduced, it has 
been said, from analyses of two volumes of their gaseous com- 
pounds. But we are acquainted with very few gaseous com- 
pounds of a large number of the elements ; nevertheless a number 
is assigned to every element purporting to be the combining 
number of that element. How are these numbers obtained ? 
In the year 1819 Dulong and Petit announced, as the result 
of experiment, that the specific heats of a number of elementary 
bodies are in an inverse ratio to the. combining numbers of 
these bodies. 
The combining numbers of mercury, sulphur, phosphorus, 
bismuth, &c., as deduced from analyses of gaseous compounds, 
* I cannot hero discuss the opposing theories of varying and unvarying 
valency. 
