CHEMISTRY 49 



gaseous combustion consequent upon their growth. It is 

 found combined with potash and lime in the form of salts 

 called oxalate of potash and lime : it is one of the most impor- 

 tant of the organic bodies. 



Crystalized oxalic acid is, in transparent bodies, intensely- 

 sour, and very poisonous. This acid is not found in the soil, 

 nor in the waters which reach the roots of plants : the simple 

 process by which it is elaborated in the interior of plants will 

 be described hereafter. Oxalic acid neutralizes alkalies per- 

 fectly, and forms several important salts. There exists a rela- 

 tion between carbonic acid, carbonic oxide and oxalic acid, 

 which will be described under the head of vegetable physi- 

 ology, 



LIGHT CARBURETTED HYDROGEN. 



This is a light, inflammable gas, which is formed by the 

 decomposition of organic substances at high temperature : in 

 warm weather it may be seen rising in bubbles from marshy 

 places and stagnant pools, when vegetables are in process of 

 decomposition. This gas is colorless, destitute of taste or odor, 

 about half the weight of common air: a lighted taper is 

 extinguished by it, while the gas ignites and burns with a pale 

 yellow flame : animals immersed in this gas cease to breathe 

 almost instantly. This is the gas which exists in marshes 

 under the name of marsh (fas, and also in coal mines under 

 the name of fire damp: violent explosions sometimes took 

 place in coal mines by the ignition of this gas mixed with 

 oxygen, from the miners' lamps, previous to the invention of 

 the safety lamp by Dr. Davy. It consists of one equivalent 

 of carbon and two of hydrogen. 



This gas, together with carbonic acid, is given off during 

 the fermentation of compost, and all large collections of vege- 

 table matter. " It is supposed, [says Johnston,] by many, to 

 minister to the nourishment of plants: it is, however, very 



o 



