Proceedings of the Polytechnic Association. 871 



return to the spherical shape it possessed before being stretched, and 

 this explains the paradox of its becoming shortened by increased 

 temperature ; in other words, contracting instead of expanding by 

 heat. 



Double Refeactiox. 

 Professor Tyndall, in a recent lecture " On Light," at the Royal 

 Institution of Londdn, .thus vqyj clearly explains the phenomenon 

 of double refraction, according to the theory that all luminous 

 effects are the result of undulations of a highly attenuated and 

 subtle fluid which pervades all space and permeates all ponderable 

 matter : " In air, water, and well-annealed glass, luminiferous 

 ether has the same elasticity in all directions. There is nothing in 

 the molecular grouping of these substances to interfere with the per- 

 fect homogeneity of the ether. But when the w^ater crystallizes to 

 ice, the case is different ; here the molecules are constrained by their 

 proper forces to arrange themselves in a certain determined manner. 

 They are, for example, closer together in some directions than in 

 others. This arrangement of the molecules carries along with it an 

 arrangement of the surrounding ether, which causes it to possess 

 different degrees of elasticity in different directions. In a plate 

 of ice, for instance, the elasticit}^ of the ether, in a direction perpen- 

 dicular to the plane of freezing, is different from its elasticity in a 

 plane parallel to the same surface. The difference is displayed in a 

 strilving manner by Iceland spar, whicli .s crystallized carbonate of 

 lime ; and in consequence of two diUcrent elasticities, a wave of 

 light passing through the spar is divided into two, the one rapid, 

 corresponding to the greater elasticity, and the other slow, corres- 

 ponding to the lesser elasticity. Where the velocity is greatest, the 

 refraction is least, and where the velocity is least the refraction is 

 greatest. Hence in Iceland spar, as we have two waves moving with 

 different velocities, we have double refraction. This is also true of 

 the greater number of crystalline bodies. If the grouping of the 

 molecules be not in all directions alike, the ether will not be in all 

 directions equally elastic, and double refraction will infallibly result."' 



Action of tue Cuticle of Plants. 

 M. Barthelemy has applied the principle of osmosis in explaining 

 plant respiration. In plants there exists a cuticle which has a chemi- 

 cal composition and a physical constitution somewhat resembling 

 caoutchouc. It is not found at the stouiata on the under surface of 



