MILD AND CAUSTIC ALKALIES. 



139 



producing a neutral compound; and vice versa in the case of 

 reddening a blue paper by acid vapours. 



Expt. 143. To dissolve Carbonic Acid Gas in Caustic Soda. 

 The substance called caustic soda belongs to the class of antacids 

 above mentioned, being one of the best known of such bodies ; 

 when dissolved in a large quantity of water it forms a solution of 

 peculiar alkaline taste, and when in a smaller quantity, a strongly 

 corrosive and biting fluid, whence the term caustic. If the caustic 

 soda is pure, no effervescence will be produced by adding some 

 diluted hydrochloric acid to a solution of it; chemical change, 

 however, will take place, for the acid properties of the hydro- 

 chloric acid will disappear, and the alkaline ones of the caustic 

 soda ; nothing but pure common salt being formed when the acid 

 and soda are mixed in just the right quantity to " neutralise " one 

 another. 



Place some caustic soda solution in a glass, and pass a current 

 of carbonic acid gas (produced by the generator described above, 

 Expt. 100) into it for some time (fig. 

 66); the carbonic acid gas will dis- 

 solve, but at the same time will act 

 chemically, although invisibly, upon 

 the caustic soda, converting it into a 

 different substance termed carbonate 

 of soda. If to the resulting carbonate 

 of soda solution some hydrochloric 

 acid be now added, a vigorous effer- 

 vescence will ensue, thus showing one 

 great difference between caustic soda 

 and carbonate of soda, the latter 

 being produced from the former by 

 making it combine chemically with 

 carbonic acid. 



In this experiment carbonate Fig. 66. Carbonic Acid passed 

 of soda has been produced from through a solution of soda, 

 caustic soda by making carbon dioxide act chemically upon the 

 latter. In practical manufacture this operation is usually reversed ; 

 carbonate of soda is first obtained (originally as a natural mineral 

 product, more recently as "barilla" from the ashes of sea-weeds, 

 and subsequently by submitting common salt to various chemical 

 processes), and is converted into caustic soda by taking away 

 carbonic acid therefrom ; the effect of this is greatly to intensify 

 the alkaline characters of the soda, and thus to render it more 

 chemically active and better fitted for preparing soap and for other 

 analogous uses. The alchemists applied the term mild alkali to 



