FATS AND RELATED SUBSTANCES. 6 1 



hydrates, protein bodies and salts are not soluble in these liquids. The 

 saponification test is also of value in identification. Many of the fats con- 

 tain unsaturated acid groups and are therefore able to absorb certain 

 amounts of halogens from specially prepared solutions. Oleic acid, 

 CisH w O 2 , absorbs iodine or bromine to form CisHsJ-Oz or CisH^BrzOz. 

 The fat to be examined is dissolved in chloroform and treated with the 

 standard solution. After a time the excess of iodine (or bromine) is de- 

 termined and that absorbed by the fat is a measure of the non-saturated 

 acid present. Linoleic acid absorbs twice as much iodine or bromine as 

 oleic acid, as the formula Ci S H 3 2O 2 becomes GsHsJiOa. 



The determination of the amount of insoluble fatty acids furnished by 

 a given weight of fat is also a valuable factor in the study of these bodies. 

 In another method the fat is saponified, the soap formed decomposed by 

 dilute sulphuric acid and the resultant product subjected to distillation. 

 Fatty acids which are volatile pass over and collect in the distillate, where 

 their amount may be determined in terms of KOH or NaOH by titration. 

 In this treatment stearin and palmitin yield acids not volatile with steam. 

 This is a valuable test and is applied in the examination of butter sup- 

 posed to be adulterated with other fats. 



LECITHIN. This is a peculiar complex body which con- 

 tains phosphoric acid and an organic basic group in place of 

 one of the fatty acid radicals in the common fats. It is 

 found in the vegetable kingdom, but commonly and most char- 

 acteristically in many animal tissues, in the brain and nerve 

 tissue, blood, lymph, milk, pus, yolk of egg, etc. It is most 

 readily prepared from the last named substance. The follow- 

 ing formula represents the supposed structure of the body. 



f O CisH0 



6 1 O PO HO O C 2 H 4 - N { ^Jp) 3 



in which distearylglycero-phosphoric acid is combined with 

 the base choline : 



It appears that several forms of lecithin exist, containing 

 oleic and palmitic acid as well as stearic. They undergo 

 saponification, yielding fatty acids, glycero-phosphoric acid and 

 choline. They are soluble in ether and alcohol and in other 

 respects resemble the true fats. With water lecithin swells 

 to a gelatinous mass which under the microscope presents a 



