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:to contain an excess of acid. CaUined gypsum is Plaster 

 of Paris which is used for making models and casts. 



944 ; The effect of gypsum on Usar soil is very charac- 

 teristic. If a good soil be mixed with a considerable volume 

 of water and the muddy liquid be then poured over a filter, 

 the water extract will come through rapidly. This, however, 

 is not the case with Usar soil. An aqueous extract of 

 Usar soils takes several hours or even days to pass through. 

 The effect of small quantities of gypsum and some other 

 salts, such as Calcium Chloride or Barium Chloride, added to 

 the muddy mixture of Usar soil and water, is the curdling 

 together of the soil particles, and the rendering of the soil 

 more permeable. The Sodium Carbonate which is the most 

 harmful substance present in Usar soil is also replaced by 

 sodium sulphate which is less harmful, 



945. Gypsum also exerts a powerful action in setting 

 free potash which has been absorbed and fixed by the earth, 

 that is to say, by double silicates in the earth. The CaO of 

 the gypsum is fixed in the soil, while a corresponding quantity 

 of K 2 O goes into solution, as may be seen from the following 

 equation : 



A1 2 3 \ 



CaO, S0 3 = *Q ,-*SiO 2 4- K 2 O, SO 3 . 

 H 2 ) 



Gypsum sets free magnesia and ammonia in the 

 same manner as potash, for the use of the crop. MgSO 4 acts 

 in the same way as gypsum as a fertilizer. 



947. Gypsum scattered on moist places in cow-houses 

 and stables, does excellent service by checking the fermenta- 

 tion of urine and by absorbing some of the odours which arise 

 from it. The ammonium sulphate produced by the addition 

 of gypsum is a more non-volatile substance than the ammo- 

 nium carbonate which is evolved from putrefying urine. The 

 reaction is expressed by the following formula: 



(NH 4 >2 CO* + CaSO 4 = CaCO 3 +(NH 4 ) 2 SO 4 . 



