Part 59

The comparative refractive power of fats can also be observed by means of the oleorefractometer of Amagat-Jean or the differential refractometer of Zune.

For details of the construction of these apparatus, with a description of the optical principles on which they are based, the papers above cited may be consulted. In this laboratory the instruments which have been employed are three in number, viz., Abbe’s small refractometer, Pulfrich’s refractometer using yellow light, and the oleorefractometer of Amagat-Jean. A brief description of the methods of manipulating these instruments is all that can be attempted in this manual.

=295. Refractive Index.=—Refractive index is an expression employed to characterize the measurement of the degree of deflection caused in a ray of light in passing from one transparent medium into another. It is the quotient of the sine of the angle of the incident, divided by the sine of the angle of the refracted ray.

In the case of oils which remain liquid at room temperatures, the determinations can be made without the aid of any device to maintain liquidity. In the case of fat which becomes solid at ordinary room temperatures, the determination must either be made in a room artificially warmed or the apparatus must have some device, as in the later instruments of Abbe and Pulfrich, and in the apparatus of Amagat-Jean, whereby the sample under examination can be maintained in a transparent condition. In each case the accuracy of the apparatus should be tested by pure water, the refractive index of which at 18° is 1.333. The refractive index is either read directly on the scale as in Abbe’s instrument, or calculated from the angles measured as in Pulfrich’s apparatus.

=296. Abbe’s Refractometer.=—For practical use the small instrument invented by Abbe will be found sufficient. The one which has been in use for many years in this laboratory is shown in Fig. 86. The illustration represents the apparatus in the position preliminary to reading the index. In preparing the sample of oil for observation the instrument is turned on its axis until the prisms between which the oil is placed assume a horizontal position, as is seen in Fig. 87. The movable prism is unfastened and laid aside, the fixed prism covered with a rectangular shaped piece of tissue paper on which one or two drops of the oil are placed. The movable prism is replaced in such a manner as to secure a complete separation of the two prisms by the film of oiled tissue paper. A little practice will enable the analyst to secure this result.

After the paper disk holding the fat is secured by replacing the upper prism, the apparatus is placed in its normal position and the index moved until the light directed through the apparatus by the mirror shows the field of vision divided into dark and light portions. The dispersion apparatus is now turned until the rainbow colors on the part between the dark and light fields have disappeared. Before doing this, however, the telescope, the eyepiece of the apparatus, is so adjusted as to bring the cross lines of the field of vision distinctly into focus. The index of the apparatus is now moved back and forth until the line of the two fields of vision falls exactly at the intersection of the cross lines. The refractive index of the fat under examination is then read directly upon the scale by means of a small magnifying glass. To check the accuracy of the first reading, the dispersion apparatus should be turned through an angle of 180° until the colors have again disappeared, and, after adjustment, the scale of the instrument again read. These two readings should nearly coincide, and their mean is the true reading of the fat under examination.

For butter fats the apparatus should be kept in a warm place, the temperature of which does not fall below 30°. For reducing the results obtained to a standard temperature, say 25°, the factor 0.000176 may be used. As the temperature rises the refractive index falls.

Example.—Refractive index of a butter fat determined at 32°.4 = 1.4540, reduced to 25° as follows: 32.4 -25 = 7.4; 0.000176 × 7.4 = 0.0013; then 1.4540 + 0.0013 = 1.4553.

The instrument used should be set with distilled water at 18°, the theoretical refractive index of water at that temperature being 1.333. In the determination above given, the refractive index of pure water measured 1.3300; hence the above numbers should be corrected for theory by the addition of 0.0030, making the corrected index of the butter fat mentioned at the temperature given, 1.4583.

=297. Pulfrich’s Refractometer.=—For exact scientific measurements, Pulfrich’s apparatus has given here entire satisfaction. In this instrument a larger quantity of the oil is required than for the abbe, and this quantity is held in a cylindrical glass vessel luted to the top of the prism. The method of accomplishing this and also an illustration of the refraction of the rays of light are shown in Fig. 88.

The angle i is measured by a divided circle read with the aid of a small telescope. The index of the prism of highly refractive glass N is known. The oil is seen at n. The light used is the yellow sodium ray (D). From the observed angle the refractive index of n is calculated from the formula

_____________ n = √N² - sin²i_.

For convenience the values of n for all usual values of i are computed once for all and arranged for use in tabular form. The latest model of Pulfrich’s apparatus, arranged both for liquid and solid bodies, and also for spectrometric observation is shown in Fig. 89.

When the sodium light is used it is placed behind the apparatus and the light is collected and reflected on the refractive prism by the lens N. Through H and G is secured the micrometric reading of the angle on the scale D by means of the telescopic arrangement F E. For regulating the temperature of the oil and adjacent parts, a stream of water at any desired temperature is made to circulate through L and S in the direction indicated by the arrows. The manner in which this is accomplished is shown in the cross section of that part of the apparatus as indicated in Fig. 90.

For further details of the construction and operation of the apparatus the original description may be consulted.

In case a spectrometric observation is desired the H ray, for instance, is produced by the geissler tube Q, Fig. 91. The light is concentrated and thrown upon the refractive prism by the lens P, the lens N, Fig. 89, being removed for this purpose.

Tables, for correcting the dispersion and for calculating the indices for each angle and fraction thereof, and for corrections peculiar to the apparatus, accompany each instrument.

=298. Refractive Indices of some Common Oils.=—The following numbers show the refractive indices obtained by Long for some of the more common oils. The light used was the yellow ray of the sodium flame.

Refractive Calculated Name. Temperature. index. for 25°. Olive oil (France) 26°.6 1.4673 1.4677 ” ” (California) 25°.4 1.4677 1.4678 Cottonseed oil 24°.8 1.4722 1.4721 ” ” 26°.3 1.4703 1.4709 ” ” 25°.3 1.4718 1.4719 Sesamé oil 24°.8 1.4728 1.4728 ” ” 26°.8 1.4710 1.4716 Castor ” 25°.4 1.4771 1.4773 Lard ” 27°.3 1.4657 1.4666 Peanut ” 25°.3 1.4696 1.4696

In case of the use of Abbe’s apparatus, in which diffused sunlight is the source of the illumination, the numbers obtained cannot be compared directly with those just given unless the apparatus be first so adjusted as to read with distilled water at 18°, 1.333. In this case the reading of the scale gives the index as determined by the yellow ray. The numbers obtained with Abbe’s instrument for some common oils are given below.

In the determinations the instrument was set with water at 18°, reading 1.3300, and they were corrected by adding 0.0030 in order to compensate for the error of the apparatus.

Material. Calculated for 25°. Corrected index. Lard 1.4620 1.4650 Cotton oil 1.4674 1.4704 Olive oil stearin 1.4582 1.4610 Lard stearin 1.4594 1.4624

=299. Oleorefractometer.=—Instead of measuring the angular value of the refractive power of an oil it may be compared with some standard on a purely arbitrary scale. Such an apparatus is illustrated by the oleorefractometer of Amagat-Jean, or by Zeiss’s butyrorefractometer.

In the first named instrument, Fig. 92, the oil to be examined is compared directly with another typical oil and the shadow produced by the difference in refraction is located on a scale read by a telescope and graduated for two different temperatures. The internal structure of the apparatus is shown in Fig. 93.

In the center of the apparatus a metal cylinder, A, is found carrying two plate glass pieces, C B, so placed as to form an angle of 107°. This cylinder is placed in a larger one, provided with two circular glass windows. To these two openings are fixed to the right and left, the telescopic attachments, G, V, S, E, and the apparatus M, H, Sʹ, Eʹ, for rendering the rays of light parallel. The field of vision is divided into two portions, light and dark, by a semicircular stop inserted in the collimator, and contains the double scale shown in the figure placed at H. The field of vision is illuminated by a gas or oil lamp placed at a convenient distance from the collimator. The inner metallic cylinder A is surrounded with an outer one, to which the optical parts are attached at D Dʹ by means of plane glass plates. This cylinder is in turn contained in the large water cylinder P P, carrying a thermometer in the opening shown at the top on the left. The manipulation of the apparatus is very simple. The outer cylinder is filled with water, at a temperature below 22°, the middle one with the typical oil furnished with the instrument, the cover of the apparatus carrying the thermometer placed in position and the cup-shaped funnel inserted in the cylinder A, which is at first also filled with the typical oil. The whole system is next brought slowly to the temperature of 22° by means of the lamp shown in Fig. 92. The telescope is adjusted to bring the scale of the field of vision into focus and the line dividing the light and shadow of the field should fall exactly on 0°a. If this be not the case the 0° is adjusted by screws provided for that purpose until it is in proper position. The typical oil is withdrawn from A by the cock R, the cylinder washed with a little of the oil to be examined and then filled therewith. On again observing the field of vision the line separating the shadow from the light will be found moved to the right or left, if the oil have an index different from that of the typical oil. The position of the dividing line is read on the scale.

For fats the temperature of the apparatus is brought exactly to 45° and the scale 0°b is used. In other respects the manipulation for the fats is exactly that described for oils. In the use of 0°a, in case the room be warmer than 22°, all the liquids employed should be cooled below 22° before being placed in the apparatus. It is then only necessary to wait until the room temperature warms the system to 22°. In the case of fats it is advisable to heat all the liquids to about 50° and allow them to cool to 45° instead of heating them to that temperature by means of the lamp.

One grave objection to this instrument is found in the absence of the proper scientific spirit controlling its manufacture and sale, as evidenced by the attempt to preserve the secret of the composition of the typical oil and the negligence in testing the scale of the instruments which will be pointed out further along.

According to Jean the common oils, when purified, give the following readings at 22°: