FATTY ACID SERIES. 33 



complete than that of the other terms ; the bodies themselves enjoy 

 an extensive natural distribution, either in the isolated condition 

 or in the form of constituent residues ; they can be obtained in a 

 comparatively pure state, many of them occurring as commercial 

 products; and their properties, both as individuals and as a class, 

 have been very carefully investigated. 



(32.) Nearly all of these primary monobasic acids are capable 

 of being distributed into one or other of two principal series, 

 known as the fatty and aromatic series respectively. To the 

 fatty acid series, beginning with the formic, acetic, and pro- 

 pionic acids, I have already directed your attention. You observe 

 that each successive member of the series differs in composition 

 from its predecessor by an increment of I atom of carbon and 2 

 atoms of hydrogen. Bodies in which this difference of CH 2 pre- 

 vails are said to be homologous, and the series of fatty acids is 

 accordingly spoken of as a homologous series. It is noticeable 

 (vide preceding table of monatomic fatty acids series) that from 

 the first to the twenty-first term, the series is complete, while 

 between the twenty-first and thirtieth terms only one intermediate 

 acid is known, namely, cerotic acid, an important constituent of 

 ordinary beeswax, and especially of the so-called Chinese wax, 

 secreted by an insect of the coccus tribe. Now, while the differ- 

 .ence in composition and properties between the acids at either 

 extremity of this series is very great, that between any two or 

 three consecutive acids, more especially of those low down 

 in the list, is so slight as to be scarcely appreciable. Thus the 

 formic and acetic acids when in a state of purity are perfectly 

 mobile, strongly corrosive liquids; the butyric, valeric, and ca- 

 proic acids are thin oils ; while the palmitic, margaric, and stearic 

 acids are mild inactive solids. In comparing formic with palmitic 

 acid, which is only two-thirds of the way down the list, we 

 scarcely perceive a single point of resemblance ; but in comparing 

 formic with acetic acid, or still more decidedly in comparing pal- 

 mitic acid with the margaric and stearic acids, the difficulty is 

 rather to see the difference than the resemblance between them. 

 Moreover, between even the upper and lower members of the 

 series there is a latent similarity, and indeed certain well-marked 



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