ALKALIES. 243 



Remarks. Common ashes effervesce powerfully with acids, and 

 they easily give a solution with hot water, which affects the taste with 

 the perception of alkalinity, and the test colors with their appropriate 

 changes. 



The most familiar use of a lye in families, is in soap making, and a 

 principal cause of failure is, that the alkali is not rendered caustic by 

 the application of a sufficient quantity of good quick lime. The den- 

 sity of the solution is ascertained by the family hydrometer, an egg, 

 which floats when the solution is sufficiently dense ; but it may be 

 dense without being caustic, and if it is not caustic, it will act but 

 partially in forming soap. It should not effervesce with acids ; if it 

 does, it is proof that the carbonic acid has not been all withdrawn, 

 and it may be necessary to pass it through more lime. If it is too 

 weak from having too much water in it, this is easily removed by 

 boiling it down. The subject of saponification will be mentioned 

 again under oils, vegetable and animal. Lye has a valuable antisep- 

 tic effect, and is often used in families, as a part of poultices, and 

 also to counteract the tendency of wounds towards tetanus. 



This alkali, as it separates almost every base from acids, and as it 

 acts with great energy upon many substances, is of great utility in 

 chemistry. It is an immediate antagonist of acids, and forms salts 

 with them. 



JHkalimeter. 



This simple instrument is founded upon the fact that 100 grains 

 of pure subcarbonate of potash, are saturated by 70 of strong sul- 

 phuric acid. The acid is placed in a glass tube graduated into 100 

 equal parts, and the tube to the extent of the graduations, is then 

 filled with water. The purity of the alkali to be tried, will be as- 

 certained by the proportion of this diluted acid which it requires for 

 perfect saturation ; if there be 60 per cent, then 100 grs. will require 

 60 divisions, and so in proportion ; if pure, it will require it all. 



If we would ascertain the proportion of pure potassa in the salt, 

 then we must employ 102 grains of the acid, and dilute it with the 

 same quantity of water, requisite to fill the tube. Ure. 



This alkali is of vast importance in glass making, soap making, in 

 medicine, in domestic economy, and in various arts, and it constitutes 

 an important article of commerce, especially from the United States 

 to Europe. 



POTASSIUM. 



1. DISCOVERY by Sir H. Davy, in October, 1807.* 



* See the Bakerian lecture for that year, in the Philos. Trans. Although soda 

 has not been, as yet, described in this work, I will give the account of the discovery 

 of its decomposition in connexion with that of potassa, as the facts in the two cases 

 are very similar, and are in both perfectly intelligible. A more particular state- 

 ment of the properties of sodium will be afterwards given. 



