442 FOODS AND FOOD ADULTERANTS. 



peratures I used the hot-room of a Turkish bath establishment. In 

 order that the fats might quickly come to (he temperature of the room 

 and also be in a convenient apparatus for dropping upon the paper 

 holder they were kept in a U-shaped small tube-holder. The one arm 

 of the tube was drawn out to an almost capillary diameter, bent over at 

 the end forming a spout to facilitate the dropping of the oil upon the 

 paper receptacle. 



The apparatus is operated as follows : Fine tissue paper of rather 

 heavy body is cut into rectangular pieces 3 cm in length by 1.5 cm in 

 breadth. One of these pieces of pa; er is placed on the lower of the 

 two glass prisms of the apparatus. Two or three drops of the oil or 

 the fat are placed upon the paper and the upper prism carefully placed 

 in position so as not to move the paper from its place. In charging the 

 apparatus with the oil in this way it is placed in the horizontal position. 

 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 dis- 

 persion apparatus is now turned until the rainbow colors on the part 

 between the dark and light field have disappeared. Before doing this, 

 however, the telescope, the eye-piece 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 dark edge 

 of the field of vision falls exactly in 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 

 the scale of the instrument again read. These two readings should 

 fall closely together, and their mean is the true reading of the fat under 

 examination. 



METHOD OF DETERMINING REFHACTIVE INDEX AT TEMPERATURES ABOVE THE NOR- 

 MAL. 



The refractive index of lards, steariues, etc., can not be taken at the' 

 ordinary room temperatures. The best method of securing the desired 

 temperature is to place the instrument and samples in a room provided 

 with suitable heaters for maintaining the temperature at a constant 

 point, about 50. Some stearines may require a slightly higher temper- 

 ature. I have, in the absence of any such room in our laboratory, used 

 the hot-rooms of the Turkish bath to good ad vantage. Another method 

 suggested by Mr. Von Schweinitz has been employed. The instrument 

 is placed on the top of an air-bath maintained at a constant tempera- 

 ture. The room must also bo kept without change of temperature. The 

 instrument should bo allowed to remain on the bath for at least one 

 hour before work is commenced. If necessary it can be protected with 



