310 INFLAMMABLES. 



(h.) Discoloration from the air, fyc. All common combustibles, 

 even the floating dust in a room, will discolor this acid ; a drop of 

 oil of turpentine does it instantly ; it is decomposed by the acid, and 

 carbon developed. 



(i.) The pure acid is not rendered turbid by dilution with water. 

 The impurities are chiefly sulphate of potash, and sulphate of lead ; 

 the latter being very insoluble, is precipitated, renders the acid milky, 

 and in time subsides ; hence dilution is a means, to a certain extent, 

 of purifying the acid.* 



(/.) The acid is purified by distillation. Dr. lire's method is 

 good, and avoids the danger which was encountered in the old way. 



Arrangement. A retort of from 2 to 4 quarts capacity ; acid 1 

 pint, adopter 3 or 4 feet long, terminating in a large receiver ; apply 

 a charcoal fire to the naked retort, which should contain along with 

 the acid, a few pieces of broken glass, or some platinum wire,f or 

 platinum foil, which will prevent the heavy recoil upon the glass, 

 produced by the sudden condensation of vapor, and^ by the great 

 weight of the fluid. 



(k.) Boiling point. Acid of sp. gr. 1.850 containing 81 per cent, 

 real acid, boils at 620, and at a lower temperature, in proportion as 

 it is mingled with more water; that of sp. gr. 1.849, boils at 605, 

 and contains 80 per cent real acid ; that of sp. gr. 1.838 containing 

 real acid 75 per cent, boils at 530, &c. It is rendered stronger 

 by heating, until the acid itself rises in vapor, and if mingled with 

 combustible matter, this is burned off by heating it. 



(Z.) The freezing point. This depends on the dilution of the 

 acid. If of sp. gr. 1780,{ it congeals at 45 ; viz. with 13 degrees 

 less than causes water to freeze ; it freezes at 32, if any where be- 

 tween 1.786, and 1.775; if 1.843, or like that of commerce, it 

 freezes at 15 ; and if half water, at 36. 



When once frozen, it does not easily melt ; it sometimes forms 

 regular prismatic crystals. 



(m.) Effects on the test fluids, the same that were mentioned under 

 the general properties of acids ; infusion of litmus is very sensible, 

 and that of purple cabbage sufficiently so ; alkanet tincture, previ- 

 ously blued by a little ammonia, is instantly turned red again by a 

 drop of the diluted acid. 



* Dr. Ure, by evaporating 100 parts of sulphuric acid, in a platinum dish, obtained 

 three quarters of a part of solid matter, of which 2 (*4 ? p. 309, c.) was sulphate of 

 potash, andl sulphate of lead. Jour. Science, Vol. IV, p. 115. 



For a table of the boiling point of acid of different densities, see Henry, Vol. I, 

 p. 386, and Eng. Jour. Science, VoL IV, p. 127. 



t I have found it to succeed well without this precaution, which, however, it 

 might be advisable to take. 



t Easily brought to this specific gravity by mingling 6 1-8 parts of the acid of 

 commerce with 1 1-8 of water. Thomson's First Principles, Vol. I, p. 214. 



See Am. Jour. Vol. VI, p. 186. 



