Chemistry and Physics. 59 



tion and then through the acid, placed behind it. The two 

 liquids were then mixed, returned to the same cells and another 

 photograph taken. The difference in the mode of vibration of 

 the base, the acid and the salt is very striking, the amplitude of 

 the vibrations within the molecule of the salt being much less 

 than in that of the base. Using then the oscillation-frequencies 

 (i. e., the reciprocals of the wave lengths), of the absorbed rays 

 as abscissas and the pi-oportional thicknesses of the weakest solu- 

 tion, in millimeters, as ordinates, a curve may be drawn indicat- 

 ing both the general and the selective absorption. From the 

 results thus obtained the author draws the following deductions : 

 (1.) When the condensation of the carbon and nitrogen in the 

 molecule of a benzenoid compound or tertiary base is modified by 

 the addition thereto of an atom of hydrogen to each atom of car- 

 bon and nitrogen, the power of selective absorption is destroyed ; 

 (2.) When the condensation of the carbon atoms in quinoline is 

 modified by the combination therewith of four atoms of hydro- 

 gen, the intensity of the selective absorption is reduced, but is 

 not destroyed ; (3.) Molecules of compounds, that is to say, com- 

 posed of dissimilar atoms, vibrate as wholes or units, and the 

 fundamental vibrations give rise to secondary vibrations which 

 stand in no visible relation to the chemical constituents of the 

 molecule whether these be atoms or smaller molecules. Hence 

 it appears that a molecule is a distinct and individual particle 

 which cannot be represented by our usual chemical formulas, 

 since these only symbolize certain chemical reactions and fail to 

 express any relation between physical and chemical properties. — 

 J. Ghem. tioc, xlvii, 685, Oct., 1885. G. f. b. 



5. On the Variation of Refractive Indices with Temperature. 

 — Dufet has continued his researches on the change produced by 

 temperature in the refractive index of solid and liquid bodies, and 

 has now published the results obtained with water, fluorite, beryl, 

 carbon-disulphide, monobrom naphthalene, turpentine and alco- 

 hol. The mean index of water was first determined by the usual 

 method, a large prism of 90° being used and a Gambey theodolite 

 reading to 5". The value obtained, for the line D and at a tem- 

 perature of 20°, was 1*33292. The index was then determined 

 for thirteen other lines of the spectrum at the same temperature. 

 The variation of the index was determined for the line D only. 

 Two methods were employed, in each of which two series of 

 experiments were made. In the first or prism method, hollow 

 prisms of 90° and of 45° wei-e used in the two series. The 

 former was adjusted to minimum deviation for parallel rays and 

 the deviation noted. The cold water contained in it being re- 

 placed by hot, successive readings of the theodolite and ther- 

 mometer gave the deviation and the corresponding temperature. 

 The 45° prism received the incident light normally on one of its 

 faces ; otherwise it was similarly used. In the second method, 

 Talbot's bands were observed, using a transparent plate with 

 parallel faces immersed in water. From the displacement of 



