FATTY ACIDS. 53 



pressure ; it cannot be distilled alone. The acid is insoluble 

 in water and its salts of the alkali metals are precipitated by 

 salt. 



Palmitic Acid, C15H31COOH.— The acid is quite insoluble 

 in water ; it crystallises from alcohol in tufts of finely crystal- 

 lised needles ; and the melted acid solidifies on cooling to a pearly 

 crystalline mass. The melting point is 62° C, and it boils under 

 100 mm. pressure at 271-5° C. ; it cannot be distilled under 

 atmospheric pressure without decomposition. 



According to Hehner and Mitchell a saturated solution of 

 palmitic acid in alcohol of specific gravity 0-8183 contains 

 from 1-32 to 1-03 grammes per 100 c.c. at 0° C. 100 parts 

 of absolute alcohol at 19-5° dissolve 9-32 parts; it is, how- 

 ever, readily soluble in boiling alcohol and crystallises out on 

 cooling. 



Stearic Acid, Cj^Hj^COOH.— This acid is quite insoluble in 

 water ; it crystallises from alcohol in white, nacreous laminae, 

 melting at 69-2° {Heintz) or 68-5° (Hehner and Mitehell) to a 

 colourless liquid, which on cooling solidifies to a crystalline 

 whitish mass. Under 100 mm. pressure it boils at •2!>1' C. It 

 cannot be distilled under the atmospheric pressure without 

 decomposition. 



Hehner and Mitchell have shown that at 0" ('. a saturated 

 solution in alcohol of 0-8183 specific gravity contains from 0-142 

 to 0-158 gramme per 100 c.c. Absolute alcohol dissolves about 

 2-5 grammes per 100 c.c. 



The salts of stearic acid are, with the exception of those of the 

 alkali metals (soaps), insoluble in water and almost insoluble in 

 alcohol. The salts of palmitic acid resemble them very much. 

 The most marked difference between these two acids is the 

 difference of solubility of the magnesium salt ; that of stearic 

 acid is practically insoluble in cold alcohol, while that of palmitic 

 acid possesses a slight, but appreciable, solubility ; the presence 

 of magnesium palmitate causes, however, appreciable solubility 

 of magnesium stearate. 



All salts of stearic acid (and palmitic acid) are partly decom- 

 posed by water into basic and acid salts ; the salts of the alkali 

 metals (soaps) cannot be dissolved without becoming appre- 

 ciably alkaline. They are, however, soluble in hot alcohol 

 without decomposition, forming solutions which gelatinise on 

 cooling. 



Soaps of stearic and palmitic acids are quite insoluble in 12 per 

 cent, solution of sodium chloride. 



General Properties of Acids of the Series, C„H2„+iC00H. 

 — The following table (lY.) gives a summary of the leading pro- 

 perties of these acids : — 



