_Obs._ Good diacetate of copper yields, by careful management, at a temperature of 400° to 560° Fahr., fully one half its weight of a greenish-coloured acid, of the sp. gr. of about 1·061, containing above 50% of hydrated acetic acid, or 43% of anhydrous acid. 20 lbs. of the ordinary acetate yields 9-3/4 lbs. of this rough acid, leaving a residuum of about 6-1/2 lbs. of metallic copper mixed with a little charcoal, in the retort; the remainder (nearly 2/10ths of the acid in the acetate) being decomposed by the heat, and lost. This 9-3/4 lbs. of crude acid yields by rectification, and dividing the products, 1/2 lb. of acid of the sp. gr. 1·023; 3 lbs. of the sp. gr. 1·042; and 6 lbs. of the sp. gr. 1·065; exclusive of a little acetone which comes over with it. In the first distillation, the strongest acid is found in the third receiver, and the weakest in the first. The acid obtained in this way is always accompanied with a little fragrant pyro-acetic spirit; which renders it preferable for aromatic vinegar and perfumery. It dissolves camphor, resins, and essential oils with facility. This is one of the oldest methods of obtaining glacial acetic acid, and the product is still preferred for some purposes. It is the RADICAL VINEGAR of the alchemists, and it is that which is preferred by the perfumers. Well-dried acetate of lead, or of iron, as well as several other acetates, may be substituted for acetate of copper in the above process; but are less economical and convenient. In all cases, great care must be taken to avoid over-firing, as thereby the quantity obtained is lessened, and the quality injured. The residuum of the distillation is pyrophoric and frequently inflames spontaneously, on exposure to the air. Due caution must be therefore observed regarding it. IV. _From Wood_, by _dry distillation_. See PYROLIGNEOUS ACID. The preparation of the purified acid, by converting it into an acetate, and subsequent distillation with a strong acid, is noticed _above._ [Illustration]