USED IN CHEMISTRY. 



be transparent. Pour them together, and imme- 

 diately a turbid appearance will be presented, and 

 a white powder will fall to the bottom. This 

 powder will be found to be magnesia. The ex- 

 planation is this : the nitric acid has a greater 

 attraction for lime than it has for magnesia ; there- 

 fore, it lets fall the latter and takes up the former. 

 The substance thus thrown down is called a pre- 

 cipitate, and the process is called precipitation. 



If all the bodies presented to each other are 

 compounds, sometimes two new substances are 

 formed. Thus, if solutions of nitrate of barytes 

 and of sulphate of soda be mixed together, the 

 former being composed of nitric acid and barytes, 

 and the latter of sulphuric acid and soda, two 

 new products will be obtained, viz. nitrate of 

 soda and sulphate of barytes. For the nitric 

 acid will leave the barytes and join to the soda, 

 and the sulphuric acid will give up the soda and 

 seize the barytes. This is called double elective 

 affinity, as the former example was of single 

 elective affinity. When different substances unite 

 chemically, and form a compound, they always unite 

 in the same proportion. Thus, water, which is 

 composed of oxygen and hydrogen, always con- 

 tains the same proportion of each ; that is, we do 

 not find that in several specimens of water the pro- 

 portions of oxygen and hydrogen vary. Also, if 

 an acid and an alkali combine together, and 

 thus form a certain salt, they always unite in the 

 same proportion to form that salt ; however, they 

 will sometimes combine in another proportion to 

 form another salt ; but when substances unite in 

 more than one proportion, the second, third, &c. 

 proportions are multiples or divisors of the first. 

 This is one of the latest discoveries in chemistry, 



B 3 



