Originally Posted by Andy M.
I found the following at this site
Cider vinegar. The cider is placed in barrels (with their bung holes open) which are exposed during the summer to the heat of the sun. The acetification is completed in the course of about two years. The process of the fermentation, however, must be watched, and as soon as perfect vinegar is formed, it should be racked off into clean barrels. Without this precaution, the acetous fermentation would run into, the putrefactive, and the vinegar spoil. Cider vinegar contains no aldehyde. It contains malic acid, and therefore yields a precipitate with lead acetate. The absence of such a precipitate indicates that the alleged cider vinegar is a manufactured substitute, although a fictitious article might yield a similar precipitate.
A quick reading of the quote would seem
to be saying that cider vinegar contains
However, a closer reading shows that it is actually giving a test to determine if the cider vinegar was made from cider or is "a manufactured substitute". See this reference Vinegar History
which says, in part
Vinegar was well known to the European alchemists of the middle ages. By pouring it over lead, they made a sweet tasting substance they called "sugar of lead", which was used well into the nineteenth century to smooth and sweeten a harsh cider. Unfortunately lead acetate is also very poisonous and it caused the early death of many a European cider drinker
The web article cited by Andy M
is a quote from a book called The Dispensatory of the United States of America
. This medicinal reference book was first published in 1833 and had 26 editions, the last edition was published in 1976. The edition cited was published in 1926.