812 TRANSACnONS OF THE AlIERICAN INSTITUTE. 



He had also turned his attention to solids, but on tindini>; that the 

 refractive index of crown glass was not changed by heat he discon- 

 tinued his investigation ; but Fizean, of France, had since discovered 

 that in nearly all cases the refractive index of sohds is increased by 

 heat ; the two exceptions noted by him being crown glass, which is 

 not altered, and fluor spar, which has its refractive power diminished 

 by heat. 



Dr. Gladstone and the Rev. Pelham Dale, while experimenting 

 together, had discovered one optical property which was constant 

 under varying temperatures, and common to solid, liquid, and gaseous 

 conditions, namely, the specific refractive energy ; that is to say, the 

 refractive index, minus one, divided by the density. The specific 

 energy multiplied by the atomic weight of a body will give its refrac- 

 tive equivalent. Tables had been computed by the lecturer showing 

 the atomic weights of various elementary substances and their corres- 

 ponding refractive equivalents, which were then exhibited. Compound 

 bodies generally possess the mean refracting power of their constitu- 

 ents. By experiment it was shown that on mixing spirits and water the 

 resulting liquid gives a little more refraction than either of these sub- 

 stances before the mixture, owing to a slight contraction in bulk when 

 spirits and water are mixed, the increased refraction being due entirely 

 to increased density. The most general law arrived at from these 

 researches, is that the refraction equivalent of any substance (whether 

 a mixture or a true chemical compound) is the sum of the refractive 

 equivalents of its constituents. 



The substance of greatest dispersive power now known is melted 

 phosphorus, ifext to it stands oxychloride of vanadium, then 

 follows sulphur. It might be asked why some substances, such as 

 Iceland spar, are double refracting. Dr. Gladstone believed such 

 substances have difierent degrees of tension in difierent directions. 

 The atoms may be more free to move _ in one way than in another, so 

 that, in one direction, they ofter more resistance to the passage of 

 light. Consequently, on dissolving such substances, the property of 

 double refraction should disappear. Such, on experiment, is found to 

 be the fact ; the solutions of such crystals give only one image, which 

 is about the mean of the two seen in the solid. 



One of the most beautiful experiments made by Dr. Gladstone was 

 the formation of ice flowers. A slab of ice cut parallel to the plane 

 of freezing was placed in front of the electric light, and greatly mag- 

 nified by means of a lens. As the ice began to melt, the flowers 



