150 PKIXCIPLES OF CHEMISTRY 



always present the same composition. Whether it be taken from nature 

 and purified, or whether it be obtained from hydrogen by oxidation, or 

 whether it be separated from any of its compounds, or obtained by some 

 double decomposition it will in every case contain one part of hydrogen 

 and eight parts of oxygen. This is because water is a definite chemical 

 compound. Detonating-gas, from which it may be formed, is a simple 

 mixture of oxygen and hydrogen, although a mixture of the same 

 composition as water. All the properties of both constituent gases are 

 preserved in detonating-gas. Either one or the other gas may be 

 added to it without destroying its homogeneity. The fundamental 

 properties of oxygen and hydrogen are not found in water, and neither 

 of the gases can be added to it. But they may be evolved from it. In 

 the formation of water there is an evolution of heat ; for the decom- 

 position of water heat is required. All this is expressed by the words, 

 Water is a definite chemical compound of hydrogen with oxygen. Tak- 

 ing the symbol of hydrogen, H, as expressing a unit quantity by weight 

 of this substance, and by expressing 16 parts by weight of oxygen by O, 

 we can express all the above statements by the chemical symbol of 

 water, H O. As only definite chemical compounds are denoted by 

 formulae, having denoted the formula of a compound substance, we 

 express by it the entire series of conceptions which are connected with the 

 representation of a definite compound, and, at the same time, the quan- 

 titative composition of the substance by weight. Further, as we shall 

 afterwards see, formulae express the volume of the gases contained in a 

 substance. Thus the formula of water shows that it contains two volumes 

 of hydrogen and one volume of oxygen. Besides which, we shall learn 

 that the formula expresses the density of the vapour of a compound, 

 and on this, as we have seen, many properties of substances depend. 

 This vapour density, as we shall learn, also determines the quantity of 

 a substance entering into a reaction. Thus the letters H 2 O tell 

 the chemist the entire history .of the substance. This is an inter- 

 national language, which endows chemistry with a simplicity, clear- 

 ness, stability, and trustworthiness founded on the investigation of the 

 laws of nature. 



Reiser (1888), in America, by employing palladium hydride, and by introducing 

 various new precautions for obtaining accurate results, found the composition of water 

 to be 15'95 parts of oxygen per 2 of hydrogen. 



Certain of the latest determinations of the composition of water are hardly less exact 

 than the analysis made by Dumas, and always give less than 8, and on the average 

 7'98, of oxygen per 1 part of hydrogen. At present, therefore, the atomic weight of 

 oxygen is taken as 15'96. However, this figure is not to be entirely depended on, and 

 for ordinary accuracy it may be considered that O = 16. 



