AGRICULTURAL CHEMISTRY. 2§ 



48. When the learner has been through with the 

 compounds, and ascertained by their symbols and 

 numbers how each one is composed, let him turn to 

 Table 11. and examine the salts in the same way. His 

 mind will thus insensibly become familiar with the 

 subject. Let him ask himself, on the first salt, of 

 what two compounds is it made up ? Let him trace it 

 back to its two compounds, and then trace these com- 

 pounds back to their elements. Then let him take the 

 second in the same way. He will find that copperas 

 contains 36 lbs. of protoxide of iron (Fe O) to 40 lbs. 

 of sulphuric acid (S 0^), and that it consolidates in 

 itself 63 lbs. of water (7 H O) ; that is, in 139 lbs. of 

 this substance are 36 lbs. of protoxide of iron, 40 of 

 sulphuric acid, and 63 of water. If he look at the 

 third, he will find that sulphate of soda (Glauber's salt) 

 is made up of soda (Na 0), sulphuric acid (S 0^), 

 and water (10 H O), 31 lbs. of the first to 40 of the 

 second and 90 of the last, so that in 161 lbs. of the 

 crystallized salts there are 90 lbs. of water. This, as 

 in other similar cases, is called the water of crystalliza- 

 Hon. If this salt is exposed to the air, the water of 

 crystallization passes off, and what was 161 lbs. of 

 crystals becomes 71 lbs. of a white powder, but , pos- 

 sesses equal value as before. The same is true of Ep- 

 som salt ; the water of crystallization passes off, and 

 leaves a white powder, much lighter than the crystals, 

 but of equal value. 



44. In the same way, if we take up sulphate of lime 

 (plaster, gypsum), we find its symbol to be Ca 0, S 0', 

 2 H O. CaO, implies one atom of lime, 28; SO", 



