130 Chronicles of Science. [Jan.. 



M. Gerney has discovered that a supersaturated solution of 

 left-handed double tartrate of soda and ammonia does not crystallize 

 in contact -with a fragment of the same salt right-handed, and vice 

 versa. From an inactive supersaturated solution of double racemate 

 of soda and ammonia, a fragment of right-handed crystal deter- 

 mines only the precipitation of right-handed crystals ; whilst a 

 portion of the same liquid, in contact with a left-handed crystal, 

 produces a deposit of the left-handed salt. 



This supplies a simple means of separating at will from the 

 double racemate of soda and ammonia, either of its two constituent 

 salts. 



M. Niepce de Saint Victor is perseveringly continuing his 

 experiments on the photographic reproduction of colours. He has 

 now succeeded in preparing a silver plate, on which all the colours, 

 and even white and black, are capable of being impressed in the 

 camera. His sixth memoir, which has just been communicated to 

 the Academy of Sciences, contains several improvements in detail, 

 principally with the view to obtain good blacks. 



A memoir has been published by M. E. Eeichert, on the 

 different refractive powers of fluids, modified by their chemical 

 composition. It contains the results of experiments on solutions 

 of common salt of different strengths, and the proportions of salt 

 shown by optical means and by ordinary analysis agree very 

 closely. An equally satisfactory result was obtained by solutions 

 of sugar ; but with alcohol and acetic acid, the differences in the 

 refractive indices are only half as great. 



The refraction- and dispersion- equivalents of chlorine, bromine, 

 and iodine, have been examined by Dr. Gladstone, and by him 

 communicated to the British Association. The refraction-equivalent 

 of the substance is the product of its atomic weight with its specific 

 refractive index — that is, its refractive index minus one, divided by 

 its density. Its dispersion-equivalent is the difference between the 

 refraction-equivalents as calculated for the two extreme lines of the 

 spectrum, A and H. 



From the determinations which Dr. Gladstone has made in con- 

 junction with the Piev. T. P. Dale, it is seen that in each case the 

 number for bromine lies between those for the other two. The 

 refraction-equivalents are, for chlorine 9'8, for bromine 15'5, and 

 iodine 24'2, and the dispersion equivalents are, for chlorine 0"5, 

 bromine 1"3, and iodine 2'6. 



Heat. — In experiments on radiant heat with the thermo-electric 

 pile, M. P. Desains proposes the employment of a differential appara- 

 tus, consisting essentially of a single source of heat, of two thermo- 

 electric piles, of a double-wire galvanometer, arid finally of a rheostat. 

 The apparatus is so arranged that the equilibrium, once obtained, 



