October 3, 1913] 



SCIENCE 



485 



anee of results, obtained under widely varying 

 conditions, bespeaks the care and patience with 

 which unavoidable errors in individual read- 

 ings have been ironed out by the law of 

 averages. 



In following the decay of phosphorescence 

 there is obviously a stage beyond which the 

 spectrophotometer, on account of its waste- 

 fulness and dispersion of light, is no longer 

 applicable. In working beyond siich limits 

 with apparatus adapted to these researches 

 from the methods of ordinary or gross photom- 

 etry. Professors Nichols and Merritt took every 

 care to excite only that band with which they 

 were at the time concerned. Where in a few 

 cases this precaution could not be observed, 

 great care was exercised to insure the desirable 

 uniqueness of significance for the results. 

 Two very important laws, established from the 

 speetrophotometric measurements, add much 

 to the significance of such measurements as 

 were of necessity made by gross photometry. 

 The laws express the individuality of behavior 

 of a given luminescence band as a whole, 

 throughout a wide range of conditions of exci- 

 tation, and, in the case of phosphorescence, 

 during decay. A band maintains measurably 

 the same relative distribution of intensities 

 and the same wave-length of maximum inten- 

 sity. As the authors express it " the band be- 

 haves as a unit." It is, now, not unreasonable 

 to assume that, beyond the limits of avail- 

 ability of speetrophotometric methods, a given 

 luminescence band still behaves as a unit, and 

 that the measurements made thereafter on the 

 band as a whole should indicate with fair cer- 

 tainty how the intensities at the individual 

 wave-lengths decay. 



Stokes's law of photo-fluorescence, namely 

 that the fluorescent light is of greater wave- 

 length than the exciting light, is verified in its 

 gross sense. But since in a large number of 

 instances, the corresponding luminescence and 

 absorption bands overlap, and since the whole 

 of a given band may be excited by light of any 

 wave-length within the region of absorption, it 

 follows that, considered in detail, the law fre- 

 quently fails. 



To these three important laws, and another 



concerning the absence of polarization in the 

 luminescence spectrum even when it is excited 

 by polarized light, the authors add, from their 

 own results and from those of other observers, 

 some general " facts connected with the decay 

 of phosphorescence." These are — the form of 

 the curve of decay; the hysteresis effect or the 

 dependence of the form of decay curve upon 

 the previous excitation to which the substance 

 has been subjected ; the effect of red and infra- 

 red rays (ingeniously used to restore a sub- 

 stance td a standard condition after each 

 excitation) ; and the effect of high tempera- 

 tures. 



Much work was done in the study of elec- 

 trical properties of fluorescent solutions. This 

 and the efforts of other investigators in the 

 same field has led generally to negative results 

 in the search for change in electrical resistance 

 of solutions during fluorescence. 



Important also is the work done to reduce 

 the initial observations, made by spectropho- 

 tometer with dift"used light of the acetylene 

 flame as a comparison standard, to the funda- 

 mental basis of energy curves, and that which 

 was done to determine the specific exciting 

 power (intensity of fluorescence excited per 

 unit of absorbed energy) of various wave- 

 lengths of exciting light. 



The last two chapters of the memoir are de- 

 voted to the consideration of theories by means 

 of which the experimental data thus far 

 gathered may be related and explained. The 

 discussion is notably interesting and lucid 

 throughout. Chapter XIV. gives an outline 

 of the dissociation theory of Wiedemann and 

 Schmidt, and shows that it has already been 

 applied with considerable success to the ex- 

 planation of fluorescence in gases. In Chapter 

 XV. the authors add such other hypotheses as 

 would seem necessary to make this theory spe- 

 cifically applicable to the problem in hand, and 

 deduce therefrom laws which follow experi- 

 mental results with surprising success — re- 

 markable, indeed, when one considers how 

 complex must be the processes which occur in 

 luminescent solutions, solid and liquid. 



One of the valuable things accomplished by 

 a memoir such as the one in review, collecting 



