Chemistry and Physics. 455 



relations between color and constitution in organic compounds. 

 The work will be of interest both to organic and physical chem- 

 ists, for the chromophore theory, the effect of substitueuts, ultra- 

 violet absorption, the recent electron theory of absorption, and 

 many other topics are treated. A section at the end of the book 

 deals with practical spectroscopic methods for the study of 

 colored substances. h. l. w. 



4. Rays of Positive Electricity. — For the past several years 

 Sir J. J. Thomson has been studying the magnetic and electro- 

 static deviation of positive rays of electricity by means of the 

 phosphorescence excited in a willemite screen by the impact of 

 the rays. Since this method did not give permanent records it 

 was not very satisfactory to the observer and it was decidedly 

 unsatisfactory, if not confusing, to the reader of the published 

 accounts of the progress of the investigation. Fortunately, 

 Thomson has worked out a photographic process which gives, of 

 course, permanent records of the loci of impact. This process 

 has the additional advantage of requiring a much shorter time of 

 exposure than was possible with a willemite screen, for the reason 

 that ordinary dry plates are much more sensitive to the rays 

 than the phosphorescent screen ; in fact, an exposure of three 

 minutes brings out, on the developed negative, curves which can- 

 not be detected with the screen. In all cases, the photographic 

 plate was placed inside the evacuated chamber and was manipu- 

 lated by suitable rods which projected outside of the receiver. 

 The ground-glass cones on the rods fitted so well into the comple- 

 mentary cones of the chamber that no leakage and consequent 

 decrease of exhaustion took place. In the final form of appa- 

 ratus employed, three independent exposures could be made by 

 lowering a long photographic plate in such a manner as to bring 

 three different areas of the plate successively in line with the 

 rays. Fifteen typical negatives are reproduced on the plate 

 accompanying the text. 



The rays after passing through the tubular cathode — in a 

 direction away from the anode — were constrained to pass 

 between the poles of a powerful electromagnet and also between 

 two parallel, electrified, metallic plates before they reached the 

 photographic plate. As usual, the directions of the magnetic 

 and electric fields were at right angles to one another. In some 

 cases these fields were coterminous, and in others the center of 

 the electric field was nearer the cathode than the axis of the 

 magnetic field. In the reproductions of the negatives the hori- 

 zontal and vertical directions uniformly correspond to the elec- 

 trostatic and magnetic deviations respectively. 



For convenience, the curves obtained on any one plate may be 

 divided into two general classes. In one class the parabolic arcs 

 are short,, of varying length, and are especially characterized by 

 starting from different points of the same vertical, geometric line. 

 This shows that the minimum electrostatic deflection suffered by 

 the particles which gave rise to the photographic impressions 



