ORGANIC ACIDS. 



38l 



(Solanin, amygdalin ( 202), and salicin, &c, are 



hyalin of worms is closely related to chitin. 

 glucosides of the vegetable kingdom. ) 



(4) Colouring Matters containing Nitrogen. 

 Their constitution is unknown, and they occur only in animals. They are in all probability 

 derivatives of haemoglobin. They are (1) hsematin ( 18, A), myohaematin ( 232, 292, a), 

 histo-haematin ( 103, IV.), and hsematoidin ( 20). (2) Bile-pigments ( 177, 3). (3) Urine- 

 pigments (except Indican). (4) Melanin C u . 2 , H 3 , N 9 . 9 , 42 . 6 or the black pigment, which 

 occurs partly in epithelium (choroid, retina, iris, and in the deep layers of epidermis in 

 coloured races) and partly in connective-tissue corpuscles (Lamina fusca of the choroid). 

 [Turacin occurs in the red feathers of Corythaix Buffoni or Plantain-Eater. Its ash contains nearly 

 6 per cent, of copper {Church). The reddish spots or parts of feathers burn with a green name."] 



II. Organic Acids free from Nitrogen. 



(1) The fatty acids, with the formula C n H. 2n _ 1 0(OH), occur in the body partly free and 

 partly in combination. Free volatile fatty acids occur in decomposing cutaneous secretions 

 (sweat). In combination, acetic acid and caproic acid occur as amido-compounds in glycin 

 ( = amido-acetic acid) and leucin ( = amido-caproic acid). More especially do they occur united 

 with glycerin to form neutral fats, from which the fatty acid is again set free by pancreatic 

 digestion ( 170, III.). 



(2) The acids of the acrylic acid series, with the formula C n H 2 _ 3 0(HO), are represented 

 in the body by one acid, oleic acid, which in combination with glycerin yields the neutral fat 

 olein. 



251. Fats. (1) Neutral fats occur very abundantly in animals, but they also occur in all 

 plants ; in the latter more especially in the seeds (nuts, almonds, cocoa nut, poppy), more 

 rarely in the pericarp (olive) or in the root. They are obtained by pressure, melting, or by 

 extracting them with ether or boiling alcohol. They [e.g., tristearin, C 57 H 110 O 6 ] contain much 

 less than the carbohydrates, such as sugar and starch ; they give a greasy spot on paper, and 

 when shaken with colloid substances, such as albumin, they yield an emulsion. When treated 

 with superheated steam or with certain ferments (p. 256, c), they take up water and yield 

 glycerin and fatty acids, and if the latter be volatile they have a rancid odour. Treated with 

 caustic alkalies they also take up water, and are decomposed into glycerin and fatty acids ; 

 the fatty acid unites with the alkali and forms a soap, while glycerin is set free. The soap- 

 solution dissolves fats. 



Glycerin is a tri-atomic alcohol, C 3 H 5 (OH) 3 , and unites with (1) the following monobasic 

 fatty acids (those occurring in the body are printed in italics): 

 Acids. Acids. 



1. Formic, ' . . CH 2 9 7. (Enanthylic, . C 7 H 14 O q 



2. Acetic, . . C 2 H 4 2 8. Gaprylic, . C 8 H 16 C>2 



3. Propionic, . . C 3 H 6 2 9. Pelargonic, . C 9 H 18 2 



4. Butyric, . . C 4 H 8 2 10. Capric, 



[Isobutyric, 



5. Valerianic, 



6. Caproic, 



C 4 H 8 2 

 C 4 H 8 2 ] 



C 6 H 122 



11. Laurostcaric, 



12. Myristic, 



13. Palmitic, 



C 12 H 24 2 

 Ci 4 H 28 2 



C 16 H 322 



14. 

 15. 

 16. 



17. 



Acids. 

 [Margaric, 

 is a mixture 

 of 13 and 14.] 



Stearic, . 



Arachinic, 



Hyanic, . 



Cerotinic, 



CigH 36 2 

 C 2oH 40 2 



a 5 H 50 o 2 



C 27 H 54 2 



The acids form a homologous series with the formula C n H 2n - 1 0(OH). With every CH 2 added 

 their boiling point rises 19. Those containing most carbon are solid, and non-volatile ; those 

 containing less C (up to and including 10) are fluid like oil, have a burning acid taste, and a 

 rancid odour. The earlier members of the series may be obtained by oxidation from the later, 

 by CH 2 being removed, while C0 2 and H 2 are formed ; thus, butyric acid is obtained from 

 propionic acid. Nos. 13 and 14 are found in human and animal fat, less abundant and more 

 inconstant are 12, 11, 6, 8, 10, 4. Some occur in sweat ( 287) and in milk ( 231). Many of 

 them are developed during the decomposition of albumin and gelatin. Most of the above 

 (except 15 to 17) occur in the contents of the large intestine ( 185). 



(2) Glycerin also unites with the monobasic oleic acid, which also forms a series, whose 

 general formula is C n H 2n _ 3 0(OH) ; and they all contain 2H less than the corresponding 

 members of the fatty acid series. The corresponding fatty acids can be obtained from the oleic 

 acid series and vice versa. Oleic acid (olein-elainic acid),,C 18 H 34 2 , is the only one found in 

 the organism ; united with glycerin, it forms the fluid fat, olein. The fat of new-born 

 children contains more glyceride of palmitic and stearic acid than that of adults, which contains 

 more glyceride of oleic acid. Oleic acid also occurs united with alkalies (in soaps) and (like 

 some fatty acids) in the lecithins ( 23). If lecithin be acted on with barium hydrate, we 

 obtain insoluble stearic, or oleic, or palmitic acids and barium oleate, together with dissolved 

 neurin ( 322, b) and baric glycerin phosphate. It appears as if there were several lecithins, of 

 which the most abundant are the one with stearic acid and that with palmitin + oleic acid 

 radicle {DiaTconow). Lecithin occurs in the blood- corpuscles ( 23), semen, and nerves, while 

 neurin is constantly present in fungi. 



