132 COKKELATION OF PHYSICAL FOECES. 



state, it doubly refracts light, and the colours are shown as in 

 the instance of glass. 



Submit a series of crystals to the same examination, and 

 different figures will be formed by different crystals, bearing 

 a constant and definite relation to the structure of the partic- 

 ular crystal examined, and to the direction in which, with 

 reference to crystalline form, the ray crosses the crystal. 



In the crystallised salts of paratartaric acid, M. Pasteur 

 noticed two 'sets of crystals which were hemihedral in oppo- 

 site directions, i. e. the crystals of one set were to those of 

 the other as to their own image reflected in a mirror ; on 

 making a separate solution of each of these classes of crys- 

 tals, he found that the solution of the one class rotated the 

 plane of polarisation to the right, while that of the other 

 class rotated to the left, and that a mixture in proper propor- 

 tions of the two solutions produced no deviation in the plane 

 of polarisation. Yet all these three solutions are what is term- 

 ed isomeric, that is, have as far as can be discovered the same 

 chemical constitution. 



In the above, and in innumerable other cases, it is seen 

 that an alteration in the structure of a transparent substance 

 alters the character and effects of the transmitted light. The 

 phenomena of photography prove that light alters the struc- 

 ture of matter submitted to it ; with regard even to vision it- 

 self, the persistence of images on the retina of the eye would 

 seem to show that its structure was changed by the impact 

 of light, the luminous impressions being as it were branded 

 on the retina, and the memory of the vision being the scar of 

 such brand. The science of photography has reference main- 

 ly to solid substances, yet there are many instances of liquid 

 and gaseous bodies being changed by the action of light : thus 

 hydrocyanic acid, a liquid, undergoes a chemical change and 

 deposits a solid carbonaceous compound by the action of 

 light. Chlorine and hydrogen gases, when mixed and pre- 

 served in darkness, do not unite, but when exposed to light 

 rapidly combine, forming hydrochloric acid. 



