230 PROGRESS OF SCIENCE. 



upon us, according to Sir J. Herschel's calculation, of about 

 seventeen billions of pounds per square inch that is, 1,148,000 

 million times more than the pressure of air at the earth's 

 surface, and which is 15 Ibs. on every square inch. It may 

 be stated, as an evidence of the interaction of the sciences 

 upon one another, that the undulatory theory might be un- 

 known even now, had not the theory of sound suggested an 

 analogy as it did from the first, for this was Huygens's first 

 idea. Young, who had supplied the principle of research in 

 the case of the wave theory, did exactly the same thing in 

 reference to the HIEROGLYPHICS of Egypt, after his examina- 

 tion of the Rosetta Stone. By indicating the process to be 

 followed, he enabled Champollion to work out the decipher- 

 ment of the great mystery enveloping the history of the 

 past. 



17741862. Biot found that a great number of liquids 

 and solutions possess the property of circular or ROTATORY 

 POLARISATION that is, have their light-waves undulating 

 corkscrew fashion ; further, that the deviation of the plane of 

 polarisation that is, alteration of light and direction of light- 

 waves can reveal differences in the composition of bodies 

 where none is exhibited by chemical analysis ; explained the 

 colours produced by polarisation ; devised an apparatus for 

 measuring the rotatory power of liquids ; also found the law of 

 distances in electricity, " that the force decreases as the square 

 of distances" As a mathematician, Biot threw great light 

 upon Egyptian astronomy, and proved it to have been very 

 accurate, and at the same time very ancient. 



1 77S~ I8l2 Malus discovered (1808) the POLARISATION 

 OF LIGHT by reflection (also discovered by Huygens, and 

 rediscovered by Brewster). Malus called polarisation the 

 division of a ray into two, and light so divided polarised 

 light. A ray of light, it must be remembered, is caused by 

 the vibrations of the luminiferous ether, occurring in all 

 directions across the path of the ray. A ray of polarised 

 light is caused by these vibrations occurring in one plane only, 

 and Iceland spar (see Huygens) has the property of altering 

 the multifarious directions of light-waves into two only 

 vertically and horizontally thus causing double refraction. 



