The oleorefractometer has been extensively used in this laboratory and the data obtained thereby have been found useful. We have not found, however, the values fixed by Jean to be constant. The numbers for lard have varied from -3.0 to -10.0, and other fats have shown almost as wide a variation from the values assigned by him.
Jean states that the number for lard, determined by the oleorefractometer, is -12, and he gives a definite number for each of the common oils and fats. On trying the pure lards of known origin in this instrument, I have never yet found one that showed a deviation of -12 divisions of the scale; but I have no doubt that there are many such lards in existence. The pure normal lards derived from the fat of a single animal would naturally show greater variations in their chemical and physical properties, than a typical lard derived from the mixed fats of a great many animals. In leaf lard, rendered in the laboratory, the reading of the oleorefractometer was found to be -10°, while with the intestinal lard it was -9°. On the other hand, a lard rendered from the fat from the back of the animal showed a reading of only -3°, and a typical cottonseed oil a reading of +12°. According to the statement of Jean, a lard which gives even as low a refractive number as -9, by his instrument, would be adjudged at least one-quarter cottonseed oil.
After a thorough trial of the instrument of Jean, I am convinced that it is of great diagnostic value, but if used in the arbitrary manner indicated by the author it would lead to endless error and confusion. In other words, this instrument is of greater value in analyses than Abbe’s ordinary refractometer, because it gives a wider expansion in the limits of the field of vision, and therefore can be more accurately read, but it is far from affording a certain means of discovering traces of adulteration with other fats.
300. Variations in the Instruments.—In the use of the oleorefractometer, attention should be called to the fact that, through some negligence in manufacture, the instruments do not give, in all instances, the same reading with the same substance. Allen obtained the following data with a sample of lard examined in three instruments, viz., 4°.5, 6°, and 11°. Such wide differences in the scales of the instruments cannot fail to disparage the value of comparative determinations.
The variations in samples of known origin, when read on the same instrument, however, will show the range of error to which the determinations made with the oleorefractometer are subject. Pearmain has tabulated a large number of observations of this kind, covering 240 samples of oils.[253]
Following are the data relating to the most important oils.
| At 22°. | |||
|---|---|---|---|
| Name of oil. | Highest reading. Degrees. | Lowest reading. Degrees. | Mean reading. Degrees. |
| Almond | 10.5 | 8.0 | 9.5 |
| Peanut | 7.0 | 5.0 | 6.0 |
| Castor | 42.0 | 39.0 | 40.0 |
| Codliver | 46.0 | 40.0 | 44.0 |
| Cottonseed (crude) | 17.0 | 16.0 | 16.5 |
| ”(refined) | 23.0 | 17.0 | 21.5 |
| Lard oil | -1.0 | 0.0 | 0.0 |
| Linseed (crude) | 52.0 | 48.0 | 50.0 |
| ” (refined) | 54.0 | 50.0 | 52.5 |
| Olive | 3.5 | 1.0 | 2.0 |
| Rape | 20.0 | 16.0 | 17.5 |
| Sesamé | 17.0 | 13.0 | 15.5 |
| Sunflower | 35.0 | 35.0 | 35.0 |
| Tallow oil | -5.0 | -1.0 | -3.0 |
| Oleic acid | -33.0 | -29.0 | -32.0 |
| At 45°. | |||
| Butter | -34.0 | -25.0 | -30.0 |
| Oleomargarin | -18.0 | -13.0 | -15.0 |
| Lard | -14.0 | -8.0 | -10.5 |
| Tallow | -18.0 | -15.0 | -16.0 |
| Paraffin | 58.5 | 54.0 | 56.0 |
Fig. 94.—Butyrorefractometer.
301. Butyrorefractometer.—Another instrument graduated on an arbitrary scale is the butyrorefractometer of Zeiss. This apparatus, which resembles in some respects the instrument of Abbe, differs therefrom essentially in dispensing with the revolving prisms of Amici, whereby the chromatic fringing due to dispersion is corrected, and on having the scale fixed for one substance, in this instance, pure butter fat. The form of the instrument is shown in [Fig. 94]. The achromatization for the butter fat is secured in the prisms between which a film of the fat is placed, as in the Abbe instrument. When a fat, differing from that for which the instrument is graduated is introduced, the fringes of the dark and light portions of the field will not only be colored (difference in dispersion), but the line of separation will also be displaced (difference in refractive power). The apparatus is therefore used in the differential determination of these two properties. It must not be forgotten, however, that butter fats differ so much in these properties among themselves as to make possible the condemnation of a pure as an adulterated sample.