TARTARIC ACID IN THE GRAPE. 123 



The explanation of this process is also simple, alcohol being repre- 

 sented by C4 Hg Og. Thus — 



Alcohol = C4 H(5 O2 ") f Vinegar = C4 H3 O3 



4 of Oxygen = O4 l_J 3 of Water == Ho O, 



Sum C4H6O0J I Sum = C4 Ho Oe 



4°. Production of Vinegar hy fermentation. — When vegetable mat- 

 ters are allowed to ferment, carbonic acid is given off and vinegar is 

 formed. In such cases this acid is the result of a series of changes, du- 

 ring which that portion of the vegetable matter which has at length 

 reached the state of vinegar has most probably passed through the seve- 

 ral previous stages of grape, sugar, and alcohol. The carbonic acid, as 

 has already been explained (p. 115), is given off during the fermentation 

 of the grape sugar, and the consequent formation of alcohol. 



To simple transformations, similar to those above described, we can 

 trace the origin of the vinegar which is met with in the living juices of 

 plants, and among the products of their decay. 



II. TARTARIC ACID. 



The grape and the tamarind owe their sourness to a peculiar acid to 

 which the name o[ tartaric acid has been given. It is also present, along 

 with other acids, in the mulberry, in the berries of the sumach {rhus co- 

 riarii), and in the sorrels, and has been extracted from the roots of the 

 couch-grass and the dandelion. 



When new wine is decanted from the lees, and set aside in vats or 

 casks, it gradually deposits a hard crust or tartar on the sides of the ves- 

 sels. This substance is known in commerce by the name of argol, and 

 when purified is familiar to you as the cream of tartar of the shops. It 

 is a compound of tartaric acid with potash, and from it tartaric acid is 

 extracted for use in medicina and in the arts. The principal use of the 

 acid is in certain processes of the calico printers. 



The pure acid is sold either in the form of a white powder or of trans- 

 parent crystals, which are colourless, and have an agreeable acid taste. 

 It dissolves readily in water, and causes a violent effervescence when 

 mixed with a solution of the carbonate of potash or of soda. As it has 

 no injurious action upon the system, it is extensively used in artificial 

 soda powders and effervescing draughts. When added in sufficient 

 quantity to a solution containing potash, it causes a white crystalline 

 powder to fall, which is cream of tartar (or hitartrate ofpotash)^ and from 

 lime water it throws down a white chalky precipitate o^ tartrate of lime. 

 J3oth of these compounds are present in the grape. 



When perfectly free from water this acid consists of — 

 Carbon . . . = 36*81 or 4 atoms. 

 Hydrogen . . = 3-00 or 2 atoms. 

 Oxygen . . . = 60'19 or 5 atoms. 



100 

 It is therefore represented by the formula C4 Hg O5. 



If we compare the numbers by which the atoms of hydrogen and ox- 

 ygen in this acid are expressed, we see that these elements are not in the 

 proportion to form water, and that this substance, therefore, cannot, like 



