VII.] COMPOSITION OF PURE WATER. in 



weigh 2 grains. The proportion by weight is therefore i6 

 grains of oxygen to 2 of hydrogen; or 8 to i, as was 

 expressed above. Chemists are thus led to the condusion 

 that water consists of a combination of hydrogen and oxygen 

 in the definite proportions of 2 volumes of hydrogen to 

 I volume of oxygen, or of 2 parts by weight of hydrogen 

 to 1 6 parts by weight of oxygen. 



When a compound is resolved into its components the 

 process is called analysis.'' All the processes hitherto de- 

 scribed have been analytical processes, but to complete 

 the discussion of the subject it is necessary to show how 

 the composition of water may be demonstrated by synthesis,'^ 

 that is to say, by putting the constituents together and 

 building up the compound. The discovery of the composi- 

 tion of water was indeed made originally by synthetical, 

 and not by analytical, processes. 



Let pure hydrogen be perfectly dried, and then burnt : 

 hold over the jet of burning gas a cold dry glass jar. Fig. 

 29, and the surface becomes rapidly bedewed, the moisture 

 condensing in drops which trickle down the side, and may 

 be collected. These drops, are nothing but pure water, 

 which has been produced by the union of the burning 

 hydrogen with the oxygen of the surrounding air. Most of 

 our ordinary combustibles — such as coal, wood, oil, wax, 

 tallow, and gas — are rich in hydrogen, and they conse- 

 quently produce water during their combustion. Hold a 

 cold bright mirror near to a flame, and the moisture is 

 instantly condensed upon its surface. 



Instead of the hydrogen being burnt in air, and thus 

 caused to combine with atmospheric oxygen, let the hydro- 

 gen be mixed with pure oxygen in the proper proportions 



- Analysis, from ava, ana, again ; Xvais, Izisis, a separation. 

 ^ Synthesis, from aiiv, sun, together ; Bicns, thesis, putting. 



