Prof. Williamson on the Unit-volume ofGases. 189 



of alcohol, occupies twice the bulk of sixteen parts of oxygen, or 

 the same bulk as thirty-two parts of oxygen. 



Before the rectification of the atomic weight of oxygen, the 

 unit-volume was denned as being the volume of eight parts by 

 weight of oxygen. But retaining the principle of that original 

 definition, viz. that the unit-volume is the volume of one atom of 

 oxygen, we now use volumes double as great as the old volumes, 

 while referring to an atomic weight of oxygen double of what 

 was formerly in use, 



A more convenient way of describing the unit-volume now in 

 use is to call it the volume of one part by weight or one atom 

 of hydrogen, and this definition is now generally coming into 

 use. One important advantage of the rapidly prevailing system 

 of applying the term u two volumes " to the volume of two parts 

 by weight of hydrogen (or one molecule of free hydrogen), is 

 that we thus obtain an exceedingly easy means of calculating the 

 density of gases and vapours on the hydrogen scale. 



Thus when we say that the molecule of steam occupies two 

 volumes, we thereby assert chat H 2 0, or eighteen parts by 

 weight of steam, occupy twice as great a volume as one part by 

 weight of hydrogen, and our statement contains the data for cal- 

 culating the density of steam compared to that of hydrogen ; 

 for it amounts to saying that two volumes of steam weigh 

 eighteen, while one volume of hydrogen weighs one ; so that 

 one volume of steam weighs nine times as much as one volume 

 of hydrogen. 



Again, the molecule of chlorosulphuric acid, SO 2 CI 2 , is stated 

 to occupy two volumes in the state of vapour, so that this weight 

 (S = 32, 2 =32, Cl 2 = 71), 135, is as bulky as two parts by 

 weight of hydrogen, and our vapour has accordingly a density 

 of 67^5. 



Or when we say that a molecule of marsh -gas occupying two 

 volumes requires four atoms of oxygen, occupying four volumes, 

 for its complete combustion, and that the product is a molecule 

 of carbonic acid occupying two volumes, and two molecules of 

 steam occupying four volumes according to the equation 



CH 4 + O 4 = CO 2 + (H 2 0) 2 , 



2 vols. 4 vols. 2 vols. 4 vols. 



we similarly supply the data for calculating the density of each 

 of the aeriform bodies named. Thus CH 4 weighing sixteen, 

 and occupying two volumes, is eight times as heavy as two vo- 

 lumes of hydrogen. O 4 weighing sixty-four is of course sixteen 

 times as heavy as its own bulk of hydrogen. CO 2 weighs forty- 

 four and occupies two volumes, proving that it has a density of 

 22 ; and the density of steam is found in like manner. 



