32 THE BUTYRIC ACID GROUP. 



Valerianic acid C 10 H 9 O 3 . H O=(C H) 10 O 4 . 



Caproic acid C 12 H n O 3 . H O=(C H) 12 O 4 . 



GEnanthylic acid C 14 H 13 O 3 . H O=(C H) 14 O 4 . 



Caprylicacid C 16 H 15 O 3 . H O=(CH) 16 O 4 . 



Peiargonic acid C 18 H 17 O 3 . H O=:(C H) 18 O 4 . 



Gapricacid C 20 H 19 O 3 . H O=(C H) 20 O 4 . 



Closely approximating to them in their composition is another 

 somewhat extensive group of organic acids, the " fatty acids," 

 which, however, we shall consider separately, because they possess 

 certain distinctive characters which would interfere with the general 

 view which we propose to take of these acids. 



It is not surprising that as these acids present a perfect analogy 

 in their composition (homology), they should also present very many 

 similarities in their physical and chemical properties. They are 

 all fluid at an ordinary temperature, and, when freed as much as 

 possible from water, are mostly oleaginous ; they do not crystallise 

 and solidify at a higher temperature than 0, but are so volatile that 

 at an ordinary temperature they more or less powerfully irritate the 

 eyes and nostrils ; they are colourless, but have a peculiar burning 

 or acrid taste. They are soluble in almost every proportion in 

 water, alcohol, and ether ; they redden litmus powerfully ; they 

 may be distilled without being decomposed ; their boiling point 

 ascends with the number of the atoms of the carbo-hydrogen 

 (according to Kopp, at the rate of 19 [34-2 F.] for 2 atoms of 

 CH), and the densities of the vapours of these acids have a similar 

 relation to the number of the atoms of the carbo-hydrogen ; more- 

 over these vapours are inflammable when too much aqueous vapour 

 is not mixed with them. 



Combined with bases, these acids form salts which are for the 

 most part soluble, and some of which crystallise readily. With 

 organic haloid bases, the oxides of methyl, ethyl, amyl, and 

 lipyl, they form what are called haloid salts, which are produced 

 either by direct union of the acid and the base, or by double 

 decomposition. Almost all the compounds of the first three are liquid, 

 and extremely volatile ; their boiling point is lower by a definite 

 number of degrees than that of the corresponding acids when de- 

 prived as thoroughly as possible of water. In no class of bodies 

 have so large a number of metameric substances been hitherto found 

 as in this ; thus, for instance, metacetonic acid=C 6 H 5 O 3 . HO, 

 formiateof oxide of ethyl. =C 4 H 5 O. C 2 HO 3 , and acetate of oxide 

 of methyl = C 2 H 3 O. C 4 H 3 O 3 , containing equal numbers of the 

 atoms of the individual elements C 6 H 6 O 4 , are metameric; so 



