CONSTITUTION OF MALIC AND OXALIC ACIDS. 125 



foimed in nature, it is chiefly as a product of the decomposition of or- 

 ganic matter, when it has already ceased to exist in, or to form part of, 

 a living plant. 



Along with the citric acid, it has been already stated that the malic 

 occurs in many fruits. It is found more abundantly, however, and is the 

 chief cause of the sour taste, in the unripe apple, [hence its name malic 

 acid,] the plum, the sloe, the elderberry, the barberry, the fruit of the 

 mountain ash, and many others. It is associated with the tartaric acid 

 in the grape and in the Agave americana. 



This acid is not used in the arts or in medicine, and therefore is not 

 usually sold in the shops. It is obtained most readily, in a pure stale, 

 from the berries of the mountain ash. It forms colourless crystals, 

 which have an agreeable acid taste. It combines with potash, soda, 

 lime, and magnesia, and forms malates, and, in combination with one or 

 more of these bases, it usually occurs in the fruits and juices of plants. 

 The malate of lime is soluble, while the citrate, as already stated, is 

 nearly insoluble, in water. This malate exists in large quantity in the 

 juice of the house-leek {sempervivum tectorum),m\he Sedum telephium, 

 the Arum maculatum, and many other juicy and fleshy-leaved plants. 



When perfectly free from water, the malic acid has exactly the same 

 chemical constitution as the citric, and is represented by the same for- 

 mula C4 H2 O4. These two acids, therefore, bear the same relation 

 to each other as we have seen that starch, gum, and sugar do. They 

 are what chemists call isomeric, or are isomeric bodies. We cannot 

 transform them, however, the one into the other, by any known means, 

 though there is every reason to believe that tltey may undergo such 

 transformations in the interior of living plants. Hence probably one 

 reason also why the malic and citric acids occur associated together in 

 so many different fruits. 



V. OXALIC ACID. 



This acid has already been treated of, and its properties and cojnposi- 

 tion detailed, in a preceding lecture (Lecture III., p. 47). It forms co- 

 lourless transparent crystals, iiaving an agreeably acid taste, and it 

 effervesces with the carbonates of potash and soda, but on account of its 

 poisonous qualities, it is unsafe to administer it as a medicine. It oc- 

 curs in combination with potash in the sorrels, in rhubarb, and in the 

 juices of many lichens. Those lichens which incrust the sides of rocks 

 and trees, not unfrequently contain half their weight of this acid in com- 

 bination with lime. It can be formed artificially by the action of nitric 

 acid on starch, sugar, gum, and many other organic substances. 



When perfectly free from water, oxalic acid contains no hydrogen ; 

 but consists of — 



Carbon . . . 33-75 = 2 atoms 

 Oxygen . . . 66-25 = 3 " 



100 

 and it is represented by C2 O3. When heated with strong sulphuric 

 acid, it is decomposed and resolved into gaseous carbonic acid (CO2) and 

 carbonic oxide (CO) in equal volumes. This change is easily under- 



