1 1 CHE MIS TR Y FOR A GRIC UL TURA L S TU DENTS 



blue with solution of litmus, and titrate with the dilute sul- 

 phuric acid of known strength. Use all the precautions 

 described on p. 71, being particularly careful to boil off 

 the carbonic acid gas set free, as this, while in solution, is acid 

 to litmus. Having obtained concordant results, calculate from 

 the sulphuric acid employed the amount of sodium in the 

 carbonate of sodium taken (remembering that sulphate of 

 sodium has the formula Na2S04). Does it correspond with 

 the formula Na^COg, and if so, do the proportions of carbon- 

 ate of sodium and sulphuric acid used correspond with the 

 equation : — 



Na.CO., + H2SO4 = Na^SO^ + CO2 + H2O ? 



Although carbonic acid gas is only slightly soluble in water under 

 ordinary atmospheric pressure, there is evidence that it does combine to 

 form an unstable carbonic acid, H2O.CO2 or H2CO3. Thus its solution is 

 acid to litmus, and a bottle of aerated water, i.e. water charged with 

 carbonic acid gas under pressure, does not immediately lose all its excess of 

 the gas when opened, but the bubbles keep rising to the surface, especially 

 when the liquid is agitated or warmed, for a cons'.derable time, indicating 

 that the acid is only slowly decomposed into its anhydride and water. In 

 its instability, carbonic acid resembles silicic acid and sulphurous acid. 



Special attention must be drawn to the fact that carbonates neutralise 

 acids like free bases. The statements with respect to the use of lime in 

 promoting nitrification in the soil (p. 93) will therefore hold good for 

 chalk. Again, with regard to the incompatibility of manures, to mix chalk 

 with "superphosphate of lime" would be as undesirable as to mix lime 

 with it. 



Through some lime water pass a current of carbonic acid 

 gas. Note that the precipitate of chalk first formed gradually 

 disappears, and a clear solution is at length obtained. Heat a 

 portion of the clear liquid. Note that bubbles of carbonic 

 acid gas are set free, and simultaneously the chalk is precipi- 

 tated. To another portion add some lime water, and note that 

 this also precipitates chalk. The simplest explanation of these 

 reactions is that a soluble acid-carbonate of calcium is formed, 



