640 



NATURE 



[July 22, 1920 



dence in support of this contention. Probably the 

 most interesting section of Mr. Hiley's volume is 

 that in which he supports and elaborates the view 

 that in the great majority of cases the stem of 

 a larch is infected by the mycelium of D. calycina 

 which is living saprophytically on the dead 

 branches. This theory is not new, but it has 

 never before been subjected to so critical an 

 examination. It is a matter of common observa- 

 tion that a branch springs from the centre of a 

 canker, and it had generally been assumed that 

 death of the branch followed invasion of the stem. 

 But Mr. Hiley now produces evidence which seems 

 to prove beyond reasonable doubt that the branch 

 has always died before the canker has originated, 

 and, in fact, that the dead branch, serving as 

 food for the fungus living saprophytically, has 

 been the vehicle of infection. Such infection 

 always takes place between the end of one grow- 

 ing season and the beginning of the next, conse- 

 quently the last wood ring in the centre of a 

 canker spot on a stem is always completely 

 formed. 



Another parasite of the larch which receives 

 exhaustive treatment in the volume is Fomes 

 annosus, the common cause of heart-rot. Unlike 

 D. calycinaj it is equally common on other coni- 

 fers, and in the aggregate does a gneat deal of 

 damage. The same may be said about Armillaria 

 mellea, perhaps the most destructive single fungus 

 species with which the forester has to contend. 



The more important leaf and seedling parasites 

 are also reviewed, the volume finishing with an 

 interesting general summary and with a useful 

 bibliography. More than seventy illustrations add 

 greatly to the value of the treatise, which is indis- 

 pensable alike to the mycologist and the forester. 



The Absorption of Light by Organic 

 Compounds. 



Etudes de Photochimie. By Dr. Victor Henri. 

 P.p. vii + 218. (Paris: Gauthier-Villars et Cie, 

 1919.) 



THIS monograph is the first instalment of a 

 series in which are to be presented the 

 results of several years of work of the author and 

 his collaborators. From 1908 to the outbreak of 

 war Dr. Henri devoted his attention to the 

 experimental study of various chemical aspects 

 of the interaction between radiation and matter, 

 dealing chiefly with the absorption of light in the 

 infra-red and ultra-violet regions, with dispersion 

 in the ultra-violet, with chemical reactions 

 brought about by light, and with certain tech- 

 nical and biological aspects of the subject. In 

 NO. 2647, VOL. 105] 



1915 he went to Russia to help in scientific work: 

 in connection with the war, and towards the end 

 of 1917 began to work up the mass of data 

 accumulated in preceding years. Very few of his 

 results had been published separately — papers by 

 Bielecki, Boll, and Wurmser will, however, be 

 familiar to workers in this field. 



The present volume essentially contains the 

 results of the author's work on absorption and 

 dispersion, and is of considerable interest. Em- 

 ploying a photographic method, carefully checked, 

 and using a powerful source of ultra-violet light, 

 worked out by himself and giving a continuous 

 spectrum, he was able to measure quantitatively 

 the exact form of the absorption curve in the 

 ultra-violet for about 240 organic compounds. As 

 he points out, this represents a very considerable 

 advance, previous work being confined to the 

 mere investigation of the positions of the bands. 

 To these measurements are added a series of de- 

 terminations of dispersion in the ultra-violet, em- 

 ploying a specially designed apparatus, and a 

 number of absorption measurements in the infra- 

 red. The application of formulae developed by 

 Helmholtz and Ketteler, Drude, Lorentz, etc., has 

 enabled him to draw conclusions as to the nature 

 of the oscillators responsible for the absorption 

 of light of different wave-lengths, the damping 

 (usually very great) to which such oscillators are 

 subjected, etc. 



The more important results are as follows : 

 (a) The oscillators absorbing in the infra-red are of 

 molecular size, are atoms or fractions of atoms in 

 the mean ultra-violet, and electrons in the ex- 

 treme ultra-violet, (b) These different oscillator 

 systems are closely bound up with one another, 

 and there exist simple numerical relations between* 

 the infra-red frequency due to a chromophore and' 

 the ultra-violet frequencies in molecules contain- 

 ing such a chromophore. This, of course, was 

 previously discovered by Baly, to whose work 

 adequate reference is not made by the author. 



(c) By the application of simple rules, the absorp- 

 tion spectrum of a compound can be calculated' 

 with considerable accuracy from its constitution 

 and the characteristic infra-red frequencies of the 

 chromophores, two simple constants for each 

 infra-red absorption band being necessary. 



(d) The structure of a molecule is essentially 

 mobile. The existence of ultra-violet absorption 

 bands is an index of a labile and reactive state. 

 This, again, is in agreement with Baly's views. 



Other more speculative conclusions are perhaps 

 less justified. The experimental work appears to 

 be of a high order, and the other volumes 

 promised will be looked for with interest. 



