244 SCIENCE PROGRESS 



Malus, unhappily, was not able to continue the work so 

 brilliantly begun. His health had been undermined during 

 his military service with the Egyptian Expedition of Napoleon, 

 and within four years he died of phthisis in Paris on February 23, 

 1812. 



The next advance was made by Arago, who in 1 8 1 1 dis- 

 covered that when a ray of polarised light, after it had passed 

 perpendicularly through a quartz plate cut in a direction at 

 right angles to its axis, was analysed by a rhomb of Iceland 

 spar, two rays coloured in complementary tints were produced 

 in all positions of the rhomb. Arago noted that the appear- 

 ances presented were exactly those which would be observed 

 if the different coloured components of the incident white 

 light emerging from the quartz plate were polarised in different 

 planes. 



The further development of this discover}* - , which singu- 

 larly enough Arago did not follow up, we owe to Biot. He 

 produced the polarised light separately from different parts 

 of the spectrum, and found that the original plane of polarisa- 

 tion was rotated by the quartz through an angle proportional 

 to the thickness of the plate, that this angle was different for 

 each of the primary colours, and increased according to a 

 definite law with the refrangibility of the light. He also made 

 the further noteworthy observation that, when the quartz plates 

 were cut from different varieties of crystals, some rotated the 

 plane of polarisation to the right, others to a similar extent to 

 the left. 



Substances thus able to rotate the plane of polarisation of 

 light are termed " optically active." 



Biot passed from this to a still more fruitful discovery, 

 that a number of natural organic substances, such as oil of 

 turpentine in the liquid or gaseous condition, and solutions 

 of sugar, camphor and tartaric acid, also rotate the plane of 

 polarisation of light. This behaviour of gaseous, liquid and 

 dissolved substances showed that optical activity was not 

 necessarily connected with the crystalline state, but in certain 

 cases, at least, resided in the individual molecules of which the 

 bodies were composed. 



Biot found that, as with solid quartz, the rotation pro- 

 duced in the plane of polarisation of light by passage through 

 a liquid or dissolved active substance is dependent upon the 



