PHYSIOLOGY 
Thus, turpentine consists of colophony or resin dissolved in "oil of turpen- 
tine," itself a mixture of several terpenes. " Canada balsam," as used for 
mounting sections, consists of a resin solidified by driving off the volatile 
oil and redissolved in a more volatile solvent. The gum-resins or bal- 
sams are variable mixtures of gums and resins, with many other acci- 
dental constituents. The best known are asafetida, as distinguished 
for its disagreeable odor as are galbanum, myrrh, and frankincense, 
the chief components of incense from time immemorial, for their fragrant 
smoke. They exude from wounds in various oriental shrubs and solid- 
ify in drops and irregular masses. 
Organic acids. The organic acids are also numerous, but four pre- 
dominate. These four, oxalic, malic, tartaric, and citric acids, are all very 
widely distributed and are not infrequently associated. Oxalic acid 
(COOH COOH) is not certainly known to occur in the free state, but 
is abundant in salts of calcium, potassium-hydrogen, and magnesium. 
Calcium oxalate is found in every large group of plants except bryo- 
phytes. It crystallizes in long slender needles (raphides) or as " crystal 
sand," with two molecules of water ; or it forms large single crystals or 
crystal aggregates, of octahedral form, when it combines with six mole- 
cules of water. (See Part III, fig. 919.) Magnesium oxalate forms 
spherites. Malic acid (COOH CH 2 CHOH COOH), which is almost 
as widely distributed as oxalic, occurs in the juice of many unripe fruits, 
especially the apple, pear, cherry, etc., either free or in salts of calcium 
and potassium. Tartaric acid (COOH CHOH CHOH COOH) is 
closely allied to malic acid. It is found abundantly in the juice of 
CH 2 COOH ] 
grapes as potassium-hydrogen tartrate. Citric acid OH C COOH 
CH 2 COOH I 
occurs in the juice of many plants, being especially abundant in the 
fruits of the citrus family (lemon, lime, orange, etc.). 
Tannins. The tannins are numerous and widely distributed, occur- 
ring especially in bark, wood, leaves, fruits, and galls. They are bitter 
and astringent substances, which form insoluble compounds with pro- 
teins and gelatin, and so are used for converting hides into leather. 
Tea leaves contain 14-16 per cent or more (dry weight), various barks 
up to 40 per cent, and galls up to 60 per cent. Some substances included 
in the loose term tannins are glucosides, and such as can be made to 
yield glucose by digestion may be considered as plastic substances 
rather than wastes. 
