490 



Professor Silvanus P. Thompson 



[May 17, 



haps equally significant that as yet no inorganic substances have been 

 found which will in the liquid state rotate the light. This appears 

 to be a property possessed solely by certain compounds of carbon. 



Fig. 14. 



Fig. ir>. 



Skew-symmeti-ical arrangement ; 

 ricrht-handed. 



Skew-symmetrical arrangement ; 

 left-handed. 



Quartz fused in the blowpipe or dissolved in potash shows no trace 

 of rotatory power. 



Yet we can have little doubt that this property is bound up in 

 the yet unravelled facts of atomic and molecular structure. In the 

 case of the liquids, such as turpentine, and sugar solution, there must 

 be some skew symmetry in the grouping of atoms in the molecule to 

 produce the result. In the case of quartz, there must be a skew in 

 the building of the molecules ; there must — to borrow a phrase from 

 the architect — be an oblique bonding of the minute bricks of which its 

 transparent mass is builded. Though w^e cannot even rebuild it from 

 its solution, we know this must be so, for we can reproduce all the 

 optical phenomena which it exhibits by an actual skew building of 

 thin slices of another non-rotatory crystal. Here is an artificial ob- 

 ject (I built it myself) constructed on Reusch's plan, from sixteen 

 thin slips.of mica built up in staircase fashion — right-handedly — one 

 above the other, and set symmetrically at equal angles of 45° to one 

 another, the whole set making a cork-screw of two complete turns. 

 In the lantern it behaves just as a quartz of about 9 millimetres thick- 

 ness would do. It even gives tolerably perfect rings, as quartz does, 

 when viewed by convergent light. 



I must now pass hastily onwards to the great discovery of Fara- 

 day. Here (Fig 16) is a magnetising coil of wire M, having about 

 8300 turns, and enclosed in an iron jacket. When it is traversed by 

 a powerful electric current from the dynamo machine, it produces an 

 intense magnetic field along its axis. In this axial position lies a bar 

 of heavy glass, not quite so dense as that which Faraday himself used, 



