--------------------------+------------------+------------------ | Specific Gravity | Per cent. Fixed | at 37°·5. | Fatty Acids. --------------------------+------------------+------------------ Mutton suet | 0·90283 | 95·56 Beef suet | 0·90372 | 95·91 Fine lard | 0·90384 | 96·20 Dripping (commercial) | 0·90456 | 94·67 Mutton dripping (genuine) | 0·90397 | 95·48 Oleomargarine butter | 0·90384 | 94·34 „ „ | 0·90234 | 94·83 „ „ | 0·90315 | 95·04 „ „ | 0·90379 | 96·29 „ „ | 0·90136 | 95·60 --------------------------+------------------+---------------- It will be noticed that the fats mostly used to adulterate butter are of a lower density. Blyth regards a gravity below 0·911 (at 37°·5) as clearly pointing to the presence of foreign fatty admixture. The specific gravity determination is made by means of the areometer, or by the gravity bottle; numerous indirect methods have also been proposed. P. Casamajor[26] suggests a process for distinguishing genuine butter from oleomargarine which is based upon the fact that the density of a liquid in which a body remains in equilibrium is the density of the body itself. As the result of his investigations it was found that pure butter at 15° would be held in equilibrium by alcohol of 53·7 per cent. (sp. gr. = 0·926), and that oleomargarine would remain in equilibrium, at the same temperature, in alcohol of 59·2 per cent. (sp. gr. = 0·905). If equal volumes of alcohol of 53·7 per cent. and 59·2 per cent. (_i.e._ an alcohol of 56·5 per cent.) are taken, and a drop of melted butter and of oleomargarine are delivered upon its surface, the former will sink to the bottom and the latter will remain at the top, so long as the two globules are warm and liquid. In case the temperature of the alcohol is about 30°, the butter will solidify and also rise to the top, whereas the oleomargarine may remain liquid. On now keeping the alcohol for a short time at a temperature of 15° the oleomargarine becomes opaque, but remains at the top, while the solidified butter will sink to the bottom. If alcohol of 59·2 per cent. is employed, oleomargarine will remain at the surface and genuine butter fall to the bottom at all temperatures above 15°, and at this temperature oleomargarine will be in equilibrium. Since not over 33 per cent. of butter is usually added to oleomargarine, it is proposed to use alcohol of 55 per cent., and consider as oleomargarine any sample which does not sink at 15°.