CHEMICAL UNITS OF WEIGHT. FORMULA 73 



It will be noted that in some cases, like (2), there is no change in 

 the total volume. In others there is a shrinkage, as in (1) and (5). 

 In still others, like (3), where chlorine monoxide decomposes, 

 there is an increase in volume. In (5), in order that all the mate- 

 rials may be gaseous, the whole experiment must be done at a 

 very high temperature (and at some suitable pressure). In (4) 

 the mercuric oxide itself does not become gaseous, but decomposes, 

 so that its own relative volume cannot be given. In (6) the zinc 

 and sulphur can be combined as vapors at 1000. The product 

 (zinc sulphide) will not remain gaseous at any temperature at 

 which its volume could be measured, however, and so its volume 

 is not recorded. 



It must be kept constantly in mind that the law applies to 

 volumes in the state of gas or vapor only. There is no rule about 

 the proportions by volume required for the chemical combination 

 of liquids and solids. 



One can read these illustrations in different ways. For 

 example: (1) A given volume of steam is formed by union of the 

 same volume of hydrogen with half as great a volume of oxygen. 

 (4) Mercuric oxide, when decomposed by heating, gives two 

 volumes of mercury vapor and one volume of oxygen in every 

 three volumes of the escaping gases. (5) One volume of phos- 

 phorus vapor, together with an equivalent quantity of oxygen, 

 will give one volume of the vapor of phosphorus pentoxide, all 

 being measured at the same temperature. In fact, whenever two 

 vaporizable substances are amongst the factors and products of a 

 chemical change, their volumes thus [are either equal, or are to 

 one another in the ratio of whole numbers. 



The Standard or Unit Volume. The volumes in the~fore- 

 going paragraph are simply relative, and the statements are true 

 of gaseous volumes of any actual dimensions (large or small), 

 provided only they bear the proper relationship, such as 2 : 1, 

 1:1, or 1:5, in each case. An actual value has been chosen, 



