DISTILLED VINEGAR. 159 



it may often be removed by cautiously heating the article in the 

 flame of a Bunsen or spirit-lamp, so as to volatilise the mercury, 

 after which a little burnishing will restore the lustre, so that little 

 or no mark is visible. Overheating must be avoided, especially 

 in the case of rings in which stones are set. Instead of a lamp, 

 a hot piece of iron may be used. 



Expt. 173. To prepare White Vinegar. The purest forms of 

 vinegar consist of the substance termed acetic acid largely diluted 

 with water, and sometimes coloured up with burnt sugar. Such 

 a liquid may be prepared as follows : Dissolve an ounce of acetate 

 of soda in a tumblerful of hot water ; in another vessel dilute a 

 teaspoonful of oil of vitriol (sulphuric acid) with two or three 

 times its bulk of water. If the two fluids are now mixed, acetic 

 acid will be formed, but will not separate from the watery fluid, 

 because it is easily soluble in water. Place the whole in a retort 

 or other distilling arrangement, such as the flask with bent tube 

 and Liebig's condenser described in Expt. 36 (fig. 23), and boil 

 off two-thirds of the fluid, condensing the vapours ; the distilled 

 liquid thus obtained will be sour to the taste, owing to the 

 volatilisation of the acetic acid along with the steam, and will in 

 fact be a dilute solution of acetic acid capable of use as vinegar. 



Expt. 174. To prepare Dilute Hydrochloric Acid. Kepeat 

 the last experiment, substituting common salt for acetate of soda. 

 Hydrochloric acid will now be formed instead of acetic acid, so that 

 the distilled liquid will be a weak solution of hydrochloric acid. 

 A stronger solution may be obtained by using cold water saturated 

 with salt, and adding cautiously to the brine about one-fifth of its 

 bulk of oil of vitriol little by little, so as to avoid the danger of 

 spurting acid about by the generation of steam, owing to the 

 development of heat on diluting the oil of vitriol (Expt. 98): 

 finally, the liquid is distilled as before, the distillation being carried 

 on until little if any more fluid drops from the condenser. The 

 solution thus obtained is not a saturated solution of hydrochloric 

 acid, but is strong enough for almost all experiments requiring that 

 acid. 



Expt. 175. To prepare Hydrochloric Acid Gas. Many other 

 volatile acids can be obtained in solution in similar fashion; the 

 less water is used in proportion to the other substances, the 

 stronger is the solution obtained. In many instances the water 

 may be omitted altogether, and thus the acid itself obtained, and 

 not merely a watery solution of it. Thus, in Expt. 270, concen- 

 trated nitric acid is obtained by heating together saltpetre and oil 

 of vitriol. If common salt and sulphuric acid be thus employed, a 

 gas (hydrochloric acid gas) is evolved, which 



