183 
further, that the ratio of hydrogen to carbon is exactly twice as 
great in the one case as in the other ; that in olefiant gas, for 
instance, the carbon is to the hydrogen as six to one, whereas, 
in marsh gas it is as six to two. Or, in other words, a given 
quantity of carbon unites with either one or two proportions of 
hydrogen to form the above compounds. Dalton, whose turn 
of mind might be described as the expression of common sense 
in its mechanical aspect, explained the constitution of these two 
compounds by supposing that the first consisted of one atom 
of carbon united with one atom of hydrogen # ©, while the 
second consisted of one atom of carbon united with two atoms 
of hydrogen © % ©, the atom of carbon being considered 
to have six times the weight of the atom of hydrogen. 
He then calculated the composition of other bodies on the 
same plan ; and found, for instance, that the quantity of 
hydrogen which unites with six parts of carbon to form olefiant 
gas unites with eight parts of oxygen to form water. Hence 
water was represented by the symbol © ©, the atom of 
oxygen being considered to have eight times the weight of 
the atom of hydrogen.* The crowning point of Dalton's theory 
was reached when he discovered that the numbers which 
expressed the respective combining proportions of carbon and 
oxygen with one part of hydrogen, also expressed the propor- 
tions in which they unite with each other. Thus the ratio of 
carbon to oxygen in carbonic oxide gas was found to be as 
six to eight ; whereas in carbonic anhydride gas it was as six 
to twice eight. The former compound he considered to result 
from the union of one atom of carbon with one atom of oxygen 
• O, and the latter to result from the union of one atom 
of carbon with two atoms of oxygen O #©. Dalton 
extended the same views to the compounds of nitrogen. 
50. Dalton thus established that general principle in 
chemistry known as the law of combination in definite and 
multiple proportions. He showed that a particular number 
might be selected for every element in such a manner that 
the proportions by weight in which any two or more elements 
combined together should be always in the ratios of their 
respective numbers, or of different multiples of those numbers. 
And he accounted for this law by supposing that the elements 
unite with one another, atom to atom } and that the pro- 
portionate number accorded to each particular demand 
expresses the relative weight of its atom. Hydrogen, being 
* Watts’s Dictionary of Chemistry : “Atomic Weights.” 
Longmans & Co. 
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