392 ON LIGHT. 



(167.) It is not in crystallized bodies only that this sing- 

 ular effect is produced. Strange as it may seem that a 

 colourless, transparent, and perfectly homogeneous fluid 

 should deviate the plane of polarization of a ray passing 

 perpendicularly through it at all; still stranger that it 

 should do so constantly in one direction for the same 

 fluid, but in opposite directions for different fluids; 

 strangest of all, that even vapours should be found pos- 

 sessing the same property : such is the case. Thus, oil 

 of turpentine and its vapour turn the plane of polariz- 

 ation to the right hand, solution of sugar to the left, and 

 so for a variety of other substances.* This property has 

 been made the basis of an elegant instrument called the 

 saccharometer, by which the quantity of sugar contained 

 in a given solution is ascertained by simple inspection 

 of the tint so produced. 



(168.) Struck by the fact, apparently so singular, of a 

 "right-and-left-handedness" inherent as it were in the 

 molecules of material bodies by the correlative fact of 

 such a tendency, or so to speak idiosyncrasy, manifest- 

 ing itself in the forms of crystals and again, in quite a 

 different field of scientific research, in the action of an 

 electrified cylindrical wire on a magnetized needle 

 placed parallel to its direction, (which turns the north 

 end of the needle to the right or to the left according to 

 the direction of the current along the wire) : it early 

 occurred to the writer of these pages that it was 



* Mr Jellett, of Trinity College, Dublin, has, I am informed, 

 recently discovered a liquid which is right-handed for one end of the 

 spectrum, but left-handed for the other! 



