114 COMBINING PROPORTIONS. 



precipitated will still be to the quantity of zinc dissolved, in the 

 ratio of the combining numbers of the two metals. Lead, 

 copper, tin., or any other metal,, when it acts .like zinc as a 

 precipitant, likewise throws down equivalent quantities of 

 other metals, and takes their place in the pre-existing compound. 

 The substitution in a saline compound of one metal for another, 

 which thus occurs, without any change in the character of the 

 compound, shows how justly the combining proportions of 

 bodies are also termed their equivalent quantities or equivalents. 

 The metal displaced, and that substituted for it, have evidently 

 the same value in the construction of the compound, and are 

 truly equivalent to each other. 



The equivalent proportions of such oxides as are bases, are 

 ascertained by rinding what quantity of each saturates the known 

 combining proportion of an acid. Thus to saturate 501parts, or a 

 combining proportion of sulphuric acid, the following proportions 

 of different bases are requisite, and are equivalent in producing 

 that effect : 



Magnesia . . . .258 



Lime . . . 356 



Soda . . . .391 



Protoxide of manganese . 445 



Potash . . . .590 



Strontian .... 647 



Barytes . . . .956 



Protoxide of lead . . 1394 



Oxide of silver . . .1450 



The addition of these bodies to sulphuric acid in the above 

 proportions destroys its sour taste and other properties as an 

 acid, of which the most characteristic is that of reddening cer- 

 tain vegetable blue colours, such as litmus. The acid is said to 

 be neutralized or saturated, and the product or compound 

 formed is a neutral salt which does not alter the blue colour of 

 litmus. Of the bases mentioned, magnesia has the greatest 

 saturating power, and oxide of silver the least ; the proportion 

 of these bases necessary to saturate the same quantity of sul- 

 phuric acid being 258 of the former, and 1450 of the latter. 



Conversely the equivalent proportions of acids are the quan- 

 tities which neutralize the known equivalent of any base or 

 alkali. Thus 590 parts of potash, or a combining proportion is 

 deprived of its alkaline properties, of which the most obvious 



