116 ACIDS OF THE ACETIC SERIES. 



properties are strongest in those where n has the least value. The 

 lowest members of the series are volatile liquids, acting as powerful 

 acids ; these successively become less and less fluid, and the highest 

 members are colourless solids, closely resembling the neutral fats in 

 outward appearance. Consecutive acids of the series present but very 

 small differences of chemical and physical properties, hence the difficulty 

 of separating them : this is further increased in the animal body by 

 the fact that exactly those acids which present the greatest similarities 

 usually occur together 1 . 



The free acids are found only in small and very variable quantities 

 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 derived 

 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 powerful 

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

 considerable interest. When heated with strong sulphuric acid it is 

 decomposed into carbonic oxide and water. It is further characterised 

 by readily effecting the reduction of metallic salts, as of mercury or 

 silver, when heated with their solutions. 



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

 1883, S. 85 et seq. 



2 Kajewski, Pfliiger's Arch. Ed. xi. (1875), S. 122. 



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



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



