ACIDS. 
445 
To avoid the burnt smell and taste, the London druggists 
mix an equal measure of water with the vinegar before dis 
tillation, and draw off the original quantity. 
Vinegar of Wood, or Pyroligneous Acid. 
Vinegar has been long prepared for the calico printers, by 
subjecting wood in iron retorts to a strong heat. The fol 
lowing arrangement of apparatus has been found to answer 
well. A series of cast iron cylinders about four feet diame 
ter, and six feet long, are set in pairs horizontally in brick 
work, so that the flame of one fire may play round both. 
Both ends project a little from the brick-work. One of them 
has a cast iron plate well fitted and firmly bolted to it, from 
the centre of which an iron pipe about six inches diameter 
proceeds, and enters at a right angle the main cooling pipe. 
The diameter of this main pipe may be from 9 to 14 inches, 
according to the number of cylinders. The other end of the 
cylinder is called the mouth of the retort. This is closed by 
an iron plate smeared round its edge with clay, and secured 
in its place by wedges. The charge of wood for such cylinder 
is about 8 ewt. 
The hard woods, oak, ash, birch, and beech, are alone 
used, but fir does not answer. The heat is kept up durin, 
the day time, and the furnace is allowed to cool during the 
night. Next morning the door is opened, the charcoal re 
moved, and a new charge of wood is introduced. The 
average produet of wood vinegar, or raw pyroligneous acid, 
is thirty-five gallons. It is much contaminated with tar ; is 
of a deep brown colour; and has a sp. gr. of 1:025, so that 
it weighs about 3 cwt. ; but the residuary charcoal is found 
to weigh no more than one-fifth of the wood employed. 
The raw pyroligneous acid is rectified by a second distil 
lation in a copper still, in the body of which about twenty 
gallons of viscid tarry matter are left from every hundred 
of vinegar, and there passes over a transparent, but brown. 
vinegar, having a considerable burnt smell, and its sp. gr. is 
1•013. Its acid powers are superior to those of the best 
wine, or malt vinegar, in the proportion of three to two. 
The French now manufacture wood vinegar in a different 
apparatus, in which the gas yielded by the wood is made to 
supply a part of the heat necessary for its own distillation. 
Fig. 220, represents this apparatus. Wood, well seasoned and dried, 
is introduced into a large upright cylinder, a, made of iron plates rivetted 
together, and having on the side of its upper part a short cylindrical neck. 
An iron cover, b, is closely fitted to this pot, and then it is lifted by means