If, therefore, at any temperature between 45° and 25° values be found for the critical line, which are less than the values corresponding to the same temperature according to the table, the sample of butter may safely be pronounced to be natural, i. e., unadulterated butter. If the reading show higher numbers for the critical line the sample should be reserved for chemical analysis. A special thermometer for use in the examination of butter will be described in the section devoted to dairy products.
304. Range of Application of the Butyrorefractometer.—The extended range of the ocular scale of the refractometer, n = 1.42 to 1.49, which embraces the refractive indices of the majority of oils and fats, renders the instrument applicable for testing oils and fats and also for examining glycerol.
By reference to the subjoined table the scale divisions may be transformed into terms of refractive indices. It gives the refractive indices for yellow light for every ten divisions of the scale. The differential column Δ gives the change of the refractive indices in terms of the fourth decimal per scale division. Owing to the accuracy with which the readings can be taken (0.1 scale division) the error of the value of n rarely exceeds one unit of the fourth decimal of n.
Table of Refractive Indices.
| Scale div. | nD. | Δ. | Scale div. | nD. | Δ. |
|---|---|---|---|---|---|
| 0 | 1.4220 | 8.0 | 50 | 1.4593 | 6.6 |
| 10 | 1.4300 | 7.7 | 60 | 1.4650 | 6.4 |
| 20 | 1.4377 | 7.5 | 70 | 1.4723 | 6.0 |
| 30 | 1.4452 | 7.2 | 80 | 1.4783 | 5.7 |
| 40 | 1.4524 | 6.9 | 90 | 1.4840 | 5.5 |
| 50 | 1.4593 | 100 | 1.4895 |
The process of observation is precisely the same as that already described. In cases, however, where the critical line presents very broad fringes (turpentine, linseed oil, etc.) it is advisable to repeat the reading with the aid of a sodium flame.
305. Viscosity.—An important property of an oil, especially when its lubricating qualities are considered, is the measure of the friction which the particles exert on other bodies and among themselves, in other words, its viscosity. In the measure of this property no definite element can be considered, but the analyst must be content with comparing the given sample with the properties of some other liquid regarded as a standard. The usual method of procedure consists in determining the time required for equal volumes of the two liquids to pass through an orifice of given dimensions, under identical conditions of temperature and pressure. In many instances the viscosity of oils is determined by comparing them with water or rape oil, while, in other cases, a solution of sugar is employed as the standard of measurement.
In case rape oil be taken as a standard and its viscosity represented by 100 the number representing the viscosity of any other oil may be found by multiplying the number of seconds required for the outflow of fifty cubic centimeters by 100 and dividing by 535. If the specific gravity vary from that of rape oil, viz., 0.915, at 15°, a correction must be made by multiplying the result obtained above by the specific gravity of the sample and dividing the product by 0.915. If n be the observed time of outflow in seconds and s the specific gravity the viscosity is expressed as follows:[255]
| V = | n × 100 × s | = | n × 100 × s |
| 535 × 0.195 | 489.525 |
