AGRrCULTTRAL lUUSEUIJ, 357 



(listln^uislied by their property of effervescing with 

 acidj. 



' Magnesia is never found alone ; its distinguishing 

 character consists in aifording a bitter salt, generally 

 called Epsom salt, when combined with the vitriolic 

 acid. 



Argill is that part of clay to which this owes its pro- 

 perty of feeling soft and unctuous, and of hardening in 

 ii:-c; it is diiiicultly solubie in ac;ds, and scarce eveL* 

 effervesces with them. When combined with the vitri- 

 olic acid, it forms allun. 



SUlclous Ecu th is often found in a stony form, such 

 VL% flint or quartz, and still more frequenll}' in that of a 

 very fine sand, such as that whereof glass is made. It 

 does not effervesce, nor is it soluble in any of the com« 

 mon acids. 



To these we may add iron, i:i that imperfect state 

 in which it exists when reduced to rust, and commonly 

 called calx oi iron. ; 



Clay is of various colaur;^; for, we meet with while 

 grey, brownish red, brownish black, yellow, or bluish 

 clays, it feels smooth aud soiJTt^whatuiietu.tus : if moist, 

 it adheres to the fingers, and if' sufiiciently so, it be- 

 comes tough and ductile ; if ^l^y, it adheres more or 

 less to th:' tongue ; if thrown into water, it. gradually 

 diffuses itself through it, and slowly separates from it. 

 It does not usually effn-vesce wiUi ;^cid3 unless a strong 

 heat be applied, or that it contains U few calcareous 

 particle?, or magnesia. If heated, it hardens and burns 

 to brick. 



It consists of argill and fine sand, usually of the si» 

 "Vicious kind in various proportions, and more or less fer- 

 ruginous. The argill forms generally from 20 to 75 

 per cent of the whole mass. The sand and calx of irom 

 the remainder. These are perfectly separable by boilr 

 ing i!i stron.<x vitriolic acid. 



