ANALYSIS OP BUTTBE-FAT. 65 



Hitlierto we have only treated of the practical side of the 

 subject, that part, namely, which relates to the detection 

 and the quantitative determination of adulteration with 

 foreign fat. "We have shown that the percentage of fatty 

 acids is lower in butter than in any other animal fat, and 

 upon this difference the whole method is based. To what- 

 ever cause this difference may be due, and whatever may 

 be the names and properties of those substances which cause 

 it, the practical bearings of the method are not influenced by 

 them. But from a scientific point of view it is certainly 

 a matter of great interest to ascertain the cause of this 

 depression. 



From the somewhat crude distillation experiments quoted 

 on page 43 we concluded, in accordance with a precon- 

 ceived opinion, that this deficiency of insoluble acids was 

 due to the presence of soluble acids ; in fact, as will be 

 remembered, we were led to the determination of the in- 

 soluble acids, because by distillation we were able to separate 

 considerable percentages of volatile acids. It must be ad- 

 mitted, however, that the acidity of the distillate might 

 well have in part been due to the formation of acrylic acid, 

 by the action of sulphuric acid upon the glycerin. It was 

 suggested, seeing this weak point, by Mr. J. A. Wanklyn, 

 that butter might differ from other fats merely by its 

 being a di-glyceride, or a mixture of tri- and di-, or even 

 mono-glycerides ; and this suggestion bore no marks of im- 

 probability. 



To settle this point, it seemed desirable to get trustworthy 

 determinations of the amount of soluble acids yielded by 

 butter-fat. The direct separation by means of distillation 

 having given doubtful and inaccurate results, various indirect 

 plans of determining the quantity of these soluble acids 

 were adopted by Dr. A. Dupr^, F.R.S., Dr. Muter, and 

 ourselves. 



