i82 CLASSICS OF MODERN SCIENCE 



Thus Dalton supposes that water is formed by the union of hydro- 

 gen and oxygen, molecule to molecule. From this, and from the 

 ratio by weight of the two components, it would follow that the mass 

 of the molecule of oxygen would be to that of hydrogen as 7 1-2 to i 

 nearly, or, according to Dalton's evaluation, as 6 to i. This ratio on 

 our hypothesis is, as we saw, twice as great, namely, as 15 to i. 

 As for the molecule of water, its mass ought to be roughly expressed 

 gy 15+2=17 (taking for unity that of hydrogen), if there were no 

 division of the molecule into two ; but on account of this division it 

 is reduced to half, 8 1-2, or more exactly 8.537, as may also be found 

 directly by dividing the density of aqueous vapour 0.625 (Gay- 

 Lussac) by the density of hydrogen 0.0732. This mass only differs 

 from 7, that assigned to it by Dalton, by the difference in the values 

 for the composition of water; so that in this respect Dalton's result 

 is approximately correct from the combination of two compensating 

 errors, — the error in the mass of the molecule of oxygen, and his 

 neglect of the division of the molecule. 



