14 SUBSTANCES SOLUBLE IN PETROLEUM SPIRIT. 



16. Less-volatile Fat- Acids. If no volatile acids are present, 

 or after their separation by distillation, as directed in 15, the 

 less volatile fat-acids may be dissolved in alcohol and subjected 

 in alcoholic solution to a fractional precipitation with acetate of 

 magnesium. This salt precipitates members of the fat-acid series 

 more easily than it does oleic acid and its homologues, and of the 

 fat-acids proper of the C n H 2n 2 series, those standing highest 

 in the series (i.e. containing the largest number of carbon-atoms) 

 are precipitated first. The magnesium precipitates appear at 

 first as soon as the acetate has been added, and in that case, 

 after having been well shaken for some time, they may soon be 

 filtered off. But subsequently it becomes necessary to add strong 

 solution of ammonia, as well as the magnesium salt, to produce 

 precipitation, and to allow the mixture to stand twelve to twenty- 

 four hours in a cold place before filtering. The fractional preci- 

 pitation is so contrived that each precipitate shall weigh about 

 1 to 5 grams, and this is continued till the tolerably strongly am- 

 moniacal liquid yields no further precipitation on the addition of 

 alcoholic solution of acetate of magnesium. Each precipitate 

 must be well washed with alcohol and decomposed with hydro- 

 chloric acid. The fat-acid must be washed with water, dried, and 

 crystallized once from boiling alcohol. After carefully drying the 

 crystals the melting-point of each fraction must be taken. The 

 acids are then recrystallized repeatedly from alcohol, and the 

 melting-point again determined. (Of. 130 to 131.) 



17. Determination of Melting- Point. The following is the 

 method I adopt when I have only a small quantity of the 

 substance at my disposal. I place a minute portion on the 

 surface of mercury contained in a small beaker. This is then 

 introduced into a small cylindrical copper air-oven in such a way 

 that it does not rest on the bottom, but remains three or four 

 centimeters from it. To allow of careful observation of the 

 substance during the experiment, I use as a cover for the air- 

 oven an ordinary bottle the bottom of which has been cut off. 

 A cork, perforated for a thermometer, is then fitted into the 

 neck. The thermometer is now introduced through the perfora- 

 tion into the mercury contained in the beaker placed just beneath, 

 until the bulb is completely covered. In doing so it is desirable 

 that some of the minute fragments of fat-acid, or other substance, 

 be as near the bulb as possible. The whole is now heated over 



