IV.] THE CARBOHYDRATES. 29 



of the observer from extraneous light, is the other Nicol's prism. The 

 polariser can be rotated by means of C. In order to read off the scale, there 

 is a telescope B. In S is a small mirror which reflects the flame of a 

 movable source of light upon the nonius. Usually the instrument is made 

 for a column of Huid 220 mm. long. 



(i.) Light the movable gas-flame opposite Q. Estimate the zero-point of 

 the instrument by placing an empty tube in the instrument, and focus until 

 the lines of the cross are sharply seen. Rotate the polariser by means of C 

 until the illuminated field is seen to be traversed by dark interference-lines 

 (fig. 14, a). On rotating still further, the lines become paler, until ultimately 

 a clear space without lines occupies the field. Try to get this in the middle 

 of the field as in fig. 14, b. 



(2.) Replace the empty tube with the fluid to be investigated, when the 

 interference-lines reappear. Suppose the substance is dextro-rotatory, then 

 rotate the Mcol to the left until the lines disappear ; but from the arrange- 

 ment of the apparatus, the milled- head C is moved in the same direction as 

 the direction of rotation of the substance. It is well to make readings in all 

 four quadrants of the instrument. It is best to use the instrument in a dark 



LESSON IV. 



FATS BONE EXERCISES ON THE FOREGOING. 

 NEUTRAL FATS. 



THE neutral fats of the adipose tissue of the body generally con- 

 sist of a mixture of the neutral fats stearin, palmitin, and olein, 

 the former two being solid at ordinary temperatures, while olcin 

 is fluid, and keeps the other two in solution at the temperature 

 of the body. 



Neutral fats are derivatives of the triatomic alcohol glycerin, 



and are glycerides or compound ethers of palmitin, stearin, and 

 olein, in which three of the hydrogen atoms of the glycerin are 

 replaced by as many equivalents of the acid radical. 



I. Reactions. 



(a.) They are lighter than water; sp. gr. .91-. 94. 



(b.) Use almond or olive oil or lard, and observe that fat is 

 soluble in ether, chloroform, and hot alcohol, but insoluble m 

 water. 



(c\ Dissolve a little fat in 3 cc. ether. Let a drop o 



