VARIOUS SOLUTIONS. 47 



a, b, c, thickness 0.038, 0.023, an d 0.009 mm., respectively, of lampblack 

 are due to Angstrom. 1 The transmission curve of diamond is due to 

 Julius. 2 The lampblack is opaque to the visible and shows an increase 

 in transmission with wave-length. The diamond crystal is transparent to 

 the visible and has well-defined absorption bands at 3, 4, and 5 \i and 

 complete opacity beyond 8 /*. A band of metallic reflection is predicted 

 at 12 n, which happens to coincide with the band of carborundum. 



It seems rather remarkable that diamond should have one of its prin- 

 cipal absorption bands in the region where carbohydrates have a wide 

 band of great transparency. 



GROUP II: TRANSMISSION SPECTRA OF VARIOUS SOLUTIONS. 



The data presented here were obtained several years ago (but not pub- 

 lished) in connection with an investigation of methods of measuring radi- 

 ant efficiencies. 3 One of the methods of measuring the efficiency of an 

 illuminant, i. e., the ratio of the light emitted to the total radiation, is to 

 absorb the infra-red by means of a water cell. The idea persists even to 

 this day that a water solution of potassium alum absorbs more heat than 

 does clear water, although Donath and others demonstrated, long ago, that 

 this is not the case; the data herewith presented is further proof of this 

 fallacy. During the present year (Phys. Zeitschrift, 1907) measurements 

 of radiant efficiencies have been made using a water solution of ammo- 

 nium-iron alum, and it was claimed that the solution was more opaque 

 than pure water. A transmission curve of this substance will be shown 

 presently which indicates a greater opacity than water. The iron alum 

 oxidizes readily and it is difficult to keep the solution clear. 



The problem was recently presented to the writer by the Astrophysical 

 Observatory to suggest a substance (see asphaltum) that absorbs all the visi- 

 ble or all the infra-red energy, the dividing line being 0.76 /*. Such a sub- 

 stance would of course be an ideal energy filter; but no such substance is 

 known. Indeed, from all the substances examined, it appears that none 

 (at least none of the common ones) have large absorption bands near the 

 visible. Beryl 4 is recorded as having a large band at 0.89 //, when ex- 

 amined in polarized light. Cyanine would be a fairly good material for 

 absorbing the visible, but it is very opaque in the region of 6 to 8 ft. Io- 

 dine would be much better material for absorbing the visible, and trans- 

 mitting the infra-red. Its absorption band, at about 7.3 ft, is very weak. 

 The absorption band of iodine in the visible spectrum corresponds closely 

 with the sensibility curve of the eye. Since we are concerned only with the 

 light that affects the eye, the fairest rating of efficiencies would be to com- 



o 



1 Angstrom: Wied. Annalen, 36, p. 717, 1889. 



2 Julius: Konigkl. Akad. Wiss. Amsterdam, Deel I, No. i, 1892. 

 'Nichols & Coblentz: Phys. Rev., 17, p. 267, 1903. 



4 Konigsberger: Ann. der Phys. (3), 61, p. 687, 1897. 



