256 MR. JOHN LEIGH ON THE CHEMICAL CHANGES 



water in which these changes take place, reducing the 

 sulphates, phosphates, &c., which chiefly constitute them, to 

 sulphurets, phosphurets, &c. The metallic bases with 

 which these are ultimately left in combination, being finally 

 oxidized by the oxygen of decomposed water, unite with 

 the carbonic acid eliminated by the decaying matter, 

 forming carbonates of the earths, alkalies, &c., which origin- 

 ally existed in other forms in the vegetable fabrics, and in 

 the water. The hydrogen, liberated from the decomposed 

 water, seizes on the sulphur, phosphorus, and carbon, with 

 which it is in contact at the moment of liberation, and 

 escapes as sulphuretted hydrogen, phosphuretted hydrogen, 

 and carburetted hydrogen. A portion of the hydrogen 

 also of the decomposing plants now enters into new com- 

 binations ; part uniting with the oxygen of the plant, form- 

 ing water, and another part with its carbon, forming light 

 carburetted hydrogen, or marsh gas, or fire damp, a gas 

 composed of 2 atoms of hydrogen, and 1 atom of carbon, or 

 of carbon 2, hydrogen 4. Instead of the mere elimination 

 of carbonic acid and water, then, as takes place when vege- 

 table matter decomposes or decays with free access of air, 

 we have, when occurring under water, or in contact with 

 water, with exclusion of air, the formation and escape of 

 carbonic acid, light carburetted hydrogen, sulphuretted 

 hydrogen, phosphm-etted hydrogen, which are all gaseous, 

 and water, which remains behind. Whoever has stood over 

 a marsh or a stagnant pool, or watched a foul drain drag 

 its slow length along, has observed bubbles of gas to gurgle 

 up to the surface, float awhile and burst. The gas con- 

 tained in these bubbles, on analysis is found to consist of 

 C£^bonic acid, light carburetted hydrogen (hence called 

 marsh gas), and, when in considerable quantity, sulphuretted 

 hydrogen also. 



The pale phosphoric light which seems to enwrap masses 

 of decaying wood in the interior of trees, sometimes called 



