176 NOTES. 



and, in chemical language, is called carbonate of lime. 

 When sulphuric acid is added to it, the carbonic acid is set 

 free, and the sulphuric acid unites with the lime to form 

 sulphate of lime. 



Carbonic acid, under ordinary circumstances, is a colour- 

 less invisible gas, about half as heavy again as air. Dr. 

 Faraday first showed that under great pressure it could 

 be obtained in a liquid state. Thilorier, a French che- 

 mist, afterwards found that it could be solidified. 



LECTURE II. 



( 7 ) Page 41. Crystallisation of alum. The solution 

 must be saturated that is, it must contain as much alum 

 as can possibly be dissolved. In making the solution it 

 is best to add powdered alum to hot water as long as it 

 dissolves ; and when no more is taken up, allow the so- 

 lution to stand a few minutes and then pour it oif from 

 the dirt and undissolved alum. 



( 8 ) Page 43. Red precipitate of biniodide of mercury. 

 A little care is necessary to obtain this precipitate. The 

 solution of iodide of potassium should be added to the 

 solution of perchloride of mercury (corrosive sublimate) 

 very gradually. The red precipitate which first falls is 

 redissolved when the liquid is stirred: when a little 

 more of the iodide of potassium is added a pale red pre- 

 cipitate is formed, which, on the further addition of the 

 iodide, changes into the brilliant scarlet biniodide of mer- 

 cury. If too much iodide of potassium is added, the 

 scarlet precipitate disappears, and a colourless solution is 

 left. 



( 9 ) Page 43. Paper coated with scarlet biniodide of 

 mercury. In order to fix the biniodide on paper, it must 



