Earths. 177 



matter, which, on being exposed to the air, 

 crumbles into dust. It is to be distinguished 

 from common clay by effervescing with acids. 

 Stone marl differs from clay marl only in being 

 much harder ; but it differs from stone in break- 

 ing to pieces when exposed to the air. Most of 

 these species of calcareous earth derive their ori- 

 gin from shells. Marbles are frequently formed 

 of shells, and those of Derbyshire exhibit a re- 

 markable appearance, from their being formed of 

 the remains of a species of animals no longer, 

 known to exist. 



Lime, under . most forms, may be known by 

 effervescing with acids, and being entirely soluble 

 in some of them ; as the nitric and muriatic for 

 instance. It is proper to remark., however, that 

 farmers and other unskilful persons have some- 

 times supposed certain earths to effervesce with 

 an acid ; when what they mistook for efferves- 

 cence was the rising of the common air that was 

 separated from the mixture, which would have 

 been the case if the earth had been thrown into 

 water. We must, therefore, when we try the 

 experiment, always pour a little water on the 

 earth, which being imbibed into the pores, expels 

 the air, and makes a kind of effervescence. If, 

 after this caution, on the addition of an acid, 

 effervescence takes place, we may be sure it de- 

 pends on the presence of lime. We may know 

 whether limestone or marl is good, by observing 

 whether one-half or three-fourths dissolves in the 

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