DOMAIN XI. DECOMPOSED. 



absorbing oxygen, while it is thus convert- 

 ed into vitriolic acid ; but moisture is also 

 requisite. To this cause the decomposi- 

 tion of such stones as contain pyrites is to 

 be attributed ; it seldom acts, however, 

 unless united to some metallic substance ; 

 and hence its combinations with argil, un- 

 less assisted by heat, are not sensibly de- 

 composed, or only in a great length of 

 time. 



" Calces of iron, moderately oxygenated, 

 are the most general cause of decomposi- 

 tion, particularly when assisted by a loose 

 texture, and the other causes of disinte- 

 gration ; these act by absorbing a greater 

 proportion of oxygen and fixed air, but re- 

 quire also the assistance of moisture. By 

 this absorption they gradually swell, and 

 are disunited from the other constituent 

 parts of the stone, into whose composition 

 they enter. When least oxygenated, their 

 colour is black, or brown, or bluish ; and 

 in some instances, when united with argil 

 and magnesia, grey or greenish grey ; the 

 former in proportion as they become more 



