ELEMENTARY CHEMICAL PHYSIOLOGY 211 



an invertase (maltase) which readily hydrolyses maltose into dextrose, on 

 which the zymase of the yeast then acts, forming alcohol and carbon dioxide. 

 Another invertase in the yeast acts on cane sugar. These invertases have 

 no action on lactose. 



///. Rotation of Polarised Light. All simple carbohydrates rotate 

 the plane of polarisation of polarised light to the right except 

 laevulose, which rotates to the left. 



This effect is due to the presence in the molecule of asymmetrical carbon 

 atoms. 



6 carbon aldose (hexose). 6 carbon ketose. 



CH 2 OH CH 2 OH 



*CHOH *CHOH 



*CHOH *CHOH 



| I 



*CHOH *CHOH 



*CHOH CO 



CHO CH 2 OH 



* Denotes an asymmetrical carbon atom. 



Examination of the above formulae shows that the aldoses contain four 

 asymmetrical carbon atoms, whilst the corresponding ketoses contain only 

 three. The different arrangements in space of the hexose carbon atoms allow 

 of the existence of sixteen different hexoses, of which twelve have been 

 identified. Only two, however, are of physiological importance, dextrose and 

 galactose. The different spatial arrangement of the atoms in the molecule 

 accounts for the difference in rotatory powers shown by these aldoses and also 

 for slight differences in chemical properties, such as crystalline form and 

 melting point of the osazones. 



Polarisation of Light. When two slices of tourmaline, a semi-transparent 

 mineral, are cut parallel to the axis of the crystal and laid over one another, it 

 will be noticed that the amount of light which passes through the combination 

 varies according to the relative positions of the two slices to one another. If 

 the slices be at right angles to one another no light passes through, and in 

 intermediate positions only a certain amount, so that an opaque combination is 

 obtained. A ray of ordinary light contains vibrations in all planes passing 

 through the ray ; but when the light passes through a tourmaline plate it 

 vibrates in one plane only. Ordinary light may, therefore, be likened to a 

 wheel, the axle representing the ray of light and the spokes the planes along 

 which it vibrates. On passing through the tourmaline plate, however, the 

 light is capable of vibrating in one plane only, which would correspond, in our 

 example, to two opposite spokes. The light which vibrates in one plane is 

 called plane-polarised light, and cannot be distinguished by the naked eye 

 from ordinary light. By placing a second, similarly cut, tourmaline plate in 

 its course, however, it can be detected, for it will pass through this only if its 

 axis corresponds to the axis of the first plate. The first plate is called the 

 polariser and the second plate the analyser. The mechanism of this action of 

 the analyser and polariser can be easily illustrated by a piece of string stretched 

 between two posts ; it can vibrate in all planes. If a comb be placed in the 

 course of the string the vibrations can only take place along one plane corre- 

 sponding to the direction of the teeth of the comb. This comb represents the 

 polariser. If now, a second comb be placed along the string it will permit the 

 vibration of the string or stop it, according to the position of its teeth ; if 

 these be in the same direction as those of the first comb the string will go on 

 vibrating, but if they be placed at right angles the string will cease to vibrate. 

 Polarisation of light by tourmaline illustrates the principle of the polarimeter, 



