414 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 



f CH 2 • COOH ] 



grapes as potassium-hydrogen tartrate. Citric acid OH • C • COOH 



1 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. 



