56S Sci,iitijir Iht<M<i< nee. 



4. Tin Quantitativt Ghernical Analysis of Mixtures by Use of 

 Differences in Specific Gravity. — Hans Friedenthal has applied 

 a met bod which, in a modified form, is well known to mineralogists 

 to the analysis of complicated mixtures of organic and other sub- 

 stances, lie states that in many cases the investigation of blood, 

 milk, urine, gall, and other complex mixtures may be facilitated by 

 removing the water, grindingthe residue very fine with amechanioal 

 agate mortar, and then separating the ingredients by specific grav- 

 ity. As a liquid he uses broinoform diluted with toluol or xylol, 

 whereby a range of specific gravities from 2 -90 to O'lO may be 

 covered. A separatory funnel may be employed, or in certain 

 cases a centrifugal machine. The statement is made that a solu- 

 tion of a mixture of sodium chloride and potassium chloride may 

 be evaporated to dryness, and the two salts separated quantita- 

 tively by the use of a liquid of 2-04 specific gravity. No data are 

 given in regard to the accuracy of any of the separations, so that 

 an opinion cannot be advanced as to the value of the method. — 

 Berichte, xliv, 904. h. l. w. 



5. On Metallic Colouring in Birds and Insects. — It has been 

 thoroughly proved that the usual "flat," " dead," uniform color- 

 ing, brilliant as this sometimes can be, e. g., in birds, butterflies, 

 and flowers, finds its simple explanation in the existence of pig- 

 ment cells. On the other hand, the lively, variable " metallic " 

 glitter of burnished copper or gold ; the reflection from certain 

 aniline dyes ; the colors of certain pigeons, peacocks, humming- 

 birds, as well as a number of butterflies, beetles, and other insects, 

 require another explanation. 



After calling attention to the preceding facts, A. A. Michel- 

 son summarizes the distinguishing characteristics of "metallic" 

 reflection in the following sentences : — 



" 1. The brightness of the reflected light is always a large frac- 

 tion of the incident light, varying from 50 per cent to nearly 100 

 per cent." 



" 2. The absorption is so intense that metal films are quite 

 opaque even when their thickness is less than a thousandth of a 

 millimeter." 



" 3. If the absorption varies with color, that color which is 

 most copiously transmitted will be the part of the incident white 

 light which is least reflected — so that the transmitted light is 

 complementary to the reflected." 



" 4. The change of color of the reflected light with changing 

 incidence follows the invariable rule that the color always 

 approaches the violet end of the spectrum as the incidence 

 increases." " If the color of the normal reflexion is violet the 

 light vanishes (changing to ultra-violet), and if the normal radia- 

 tion be infra-red it passes through red, orange, and yellow as the 

 incidence increases." 



Since these four criteria are qualitative and necessary but not 

 always sufficient, the author also makes use of the more refined 

 quantitative optical tests which relate to the effects of reflection 



