140 HEATING. 



A description of the properties of these distinct t)oJ^<*5 may be 

 found in any chemical work of authority, and I only mention 

 these unions to exemplify the importance of attending to the 

 proportioJis in which bodies unite ; as we here find the very ele- 

 ments of the air we breathe, by a mere change in the pi-oportions 

 in which they are united, forming so many distinct substances, 

 from the laughing gas, nitrous oxide, up to that most powerful 

 and destructive agent, nitric acid, commonly called aqua-fortis. 



This case of the combination of nitrogen and oxygen also 

 shows the importance of the distinction between mechanical and 

 chemical union ; these two elements being only mechanically 

 united in forming atmospheric air, by which the essential prop- 

 erties of its two constituents as preserved unaltered ; whereas, 

 in the five bodies above enumerated, the union is chemical, and, 

 consequently, the essential characters of their respective con- 

 stituents are lost, and new ones obtained. 



Now, to apply these principles to the bodies under considera- 

 tion, nam.ely, the carbon and hydrogen, and ascertain the propor- 

 tions of oxygen they respectively require to produce chemical 

 union. 



These two constituents, though united in the one body — the 

 gas — yet, not only separate themselves during combustion in a 

 remarkable manner, but, by two distinct processes, form two essen- 

 tially different unions. This is an important feature of the 

 development of chemical action which the law of equivalents at 

 once points out and enables us to satisfy, although this double 

 process does not appear to be understood, much less to be pro- 

 vided for, in practice, though familiar to every chemist. 



On the first application of heat, or what may properly be 

 termed the firing or lighting the gas, when duly mixed with air, 

 the carbon separates itself from its fellmu-constituent, the hydro- 

 gen, and forms a union with the former, the produce of which 

 is carbonic acid gas. 



Now, the laws of chemical proportion teach us that carbonic 

 acid is composed of one atom of carbon vapor, (by weight 6,) 

 and two atoms of oxygen, (by weight 16,) the latter, in volume, 

 being double that of the former, as in the annexed figure : 



