16 CHEMICAL PHYSIOLOGY. 



(a.) Fill one of the decimetre tubes with distilled water, 

 taking care that no air-bubbles get in. Slip on the glass 

 disc horizontally, and screw the brass cap on the tube. 

 Place the tube in the instrument, so that it lies in the course 

 of the rays of polarised light. 



Jb.) Place some common salt (or fused common salt, and 

 ic carbonate) in the platinum spoon, and light the Bun- 

 sen'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 volatilised in the 

 outer part of the flame. In the newer form of the instru- 

 ment 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 scrupu- 

 lously clean. 



(c.) Bring the zero of the vernier to coincide with that of 

 the scale. On looking through the eye-piece, 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 to work with the field not too 

 brightly illuminated. 



(d.) 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. 



(e.) Repeat the process several times, and take the mean 

 of the readings. The difference between this reading and 

 the first at (c.), when the tube was filled with distilled water 

 i.e., zero is the rotation due to the dextrose = (a.) 



(/.) Place 10 cc. of the solution of dextrose in a weighed 

 capsule, evaporate to dryness over a water-bath, let the 

 capsule cool in a desiccator, and weigh again. The increase 



