116 ACIDS OF THE ACETIC SERIES. 



and less fluid ; and the highest members are colourless solids, 

 closely resembling the neutral fats in outward appearance. Con- 

 secutive acids of the series present but very small differences of 

 chemical and physical properties, hence the difficulty of separat- 

 ing them : this is further increased in the animal body by the 

 fact that exactly those acids which present the greatest similar- 

 ities usually occur together. 1 



The free acids are found only in small and very variable quan- 

 tities in various parts of the body ; their derivatives on the other 

 hand form most important constituents of the human frame, and 

 will be considered further on. 



Some of the lower acids of the series have been obtained by 

 treating proteids with molten caustic potash. They also occur 

 among the products of the putrefaction of proteids, as for instance 

 in old cheese. 



Of the primary alcohols from which this series of acids is de- 

 rived only two have as yet been obtained from animal tissues or 

 secretions, viz. ethyl 2 - and cetyl-alcohol, 3 C 2 H 5 . OH and C 16 H 33 . 

 OH, the former from muscle, brain, and liver, the latter in union 

 with palmitic acid in spermaceti and the secretion of the caudal 

 glands of birds. 



Formic acid. H . COOH. 



When pure is a strongly corrosive, fuming fluid, with power- 

 ful irritating odour, solidifying at C., boiling at 100 C., and 

 capable of being mixed in all proportions with either water 

 or alcohol. It has been obtained from various parts of the 

 body, such as .the spleen, thymus, pancreas, muscles, brain, and 

 blood ; in the latter its presence may be due to the action of acids 

 on the haemoglobin. It also occurs in minute traces in urine. It 

 is excreted by some ants (Formica rufa) in a fairly concentrated 

 form and may be present to the surprisingly large extent of 40 p.c. 

 in the secretion of certain caterpillars. 4 The separation of so acid 

 a fluid from the alkaline cell-substance is remarkable and of con- 

 siderable interest. When heated with strong sulphuric acid it is 

 decomposed into carbonic oxide and water. It is further charac- 

 terised by readily effecting the reduction of metallic salts, as of 

 mercury or silver, when heated with their solutions. 



Acetic Acid. CH 3 . COOH. 



It is distinguished by its characteristic odour ; its boiling-point 

 is 100 C. ; the anhydrous acid solidifies at about 17. It is solu- 

 ble in all proportions in alcohol and in water. 



1 For details on this series see Hoppe-Seyler's Hdbch. d, phys. path. chem. Anal. 

 1883, S. 85 et seq. 



2 Rajewski, Pfliiger's Arch. Bd. xi. (1875), S. 122. 



3 De Jonge, Zt.f. physiol. Chem. Bd. in. (1879), S. 225. 



4 PoultOD, The colours of animals, Internat. Sci. Ser. 1890, p. 274. 



