III.] 



THE CARBOHYDRATES. 2J 



no air-bubbles get in. Slip on the glass disc horizontally, and screw the 

 brass cap on the tube, taking care not to do so too tightly. Place the tube 

 in the instrument, so that it lies in the course of the rays of polarised light. 



(b. ) Place some common salt (or fused common salt and soda carbonate) in 

 the platinum spoon (A), and light the Bunsen's lamp, so that the soda is 

 volatilised. If a platinum spoon is not available, tie several platinum wires 

 together, dip them into slightly moistened common salt, and fix them in 

 a suitable holder, so that the salt is volatilise! in the outer part of the flame. 

 In the newer form of the instrument supplied by Laurent, there are two 

 Bunsen-burners, placed the one behind the other, which give very much 

 more light. Every part of the apparatus must be scrupulously clean. 



. 13. Wild's Polaristrobometerr 



(<. ) Bring the zero of the vernier to coincide with that of the scale. On 

 looking through the eye-piece (0), and focussing the vertical line dividing 

 the field vertically into two halves, the two halves of the field should have 

 the same intensity when the scale reads zero. If this is not the case, then 

 adjust the prisms until it is so. by means of the milled head placed for that 

 purpose behind the index dial and above the telescope tube. It is well tu 

 work with the field not too brightly illuminated. 



('/.) Remove the water-tube, and substitute for it a similar tube containing 

 the solution of the substance to be examined in this case a perfectly clear 

 solution of pure dextrose. Place the tube in position, and proceed as before. 

 The two halves of the field are now of unequal intensity. Rotate the eye- 

 piece until equality is obtained. 



(c..) Repeat the process several times, and take the mean of the readings. 

 The difference between this reading and the first at (t 1 .), when the tube 



