60 



A TEXTBOOK OF PHYSIOLOGY 



like the above, which rotate the plane of polarized light, contain one 

 or more asymmetric atoms. 



The waves of light, moving onwards, vibrate in every plane. When they pass 

 through a quartz plate they are plane polarized, and vibrate only in one plane. A 

 stretched string can be plucked and made to vibrate from side to side, or from 

 above down, or in any other plane. If the string passes through a vertical slit, 

 it can only be made to vibrate in the above-down direction; it is, so to speak, 

 plane polarized. The quartz plate has a similar effect on light. If two quartz 

 plates are arranged so that the light must pass through both, then the light polarized 

 by the first plate can pass through the second if this be placed with its optical axis 

 in the same relation as that of the first plate. If the second plate is turned at right 

 angles to the first plate, the light polarized by the first plate cannot pass the second. 

 If a solution which rotates polarized light is placed in a tube between the two quartz 

 plates, its effect on the rotation of the second quartz plate can be determined. Special 

 instruments called polarimc^ers are contrived for measuring the effect of solutions 

 on the rotation of polarized light, and so arriving at the strength of the solutions, 

 e.g., of sugar. 



The various groups may be arranged in a figure round an 

 asymmetric carbon atom so that one arrangement corresponds to 

 dextrorotation, another to levorotation. The two figures cannot be 

 superimposed so that the same two groups coincide. In this respect 

 it is interesting to note that Pasteur found that racemic acid, which 

 has holohedral crystals, and is neutral in its action to polarized 

 light, could be decomposed into dextrorotatory and levorotatory 

 tartaric acids with hemihedral crystals. Of these crystals, those 

 which were right-sided gave dextrorotation, those which were left- 

 sided gave levorotation. By growing Penicillium glaucum upon 

 racemic acid, the dextrorotatory portion of ordinary tartaric acid is 

 destroyed and the levorotatory acid is left. 



The monosaccharides are colourless crystalline bodies (with a 

 sweetish taste) readily soluble in water, with difficulty soluble in 

 alcohol, and insoluble in ether. They possess the property of depositing 

 metallic silver from" ammoniacal silver solutions, cuprous oxide from 

 alkaline copper solutions, and bismuth suboxide from alkaline bismuth 

 solutions. This property is of especial value as a test. The chief 

 tests are the following : 



1. Moore's test. Upon heating with one quarter of its volume of 

 strong caustic potash or soda, the solution becomes yellow, then brown, 



