Chemistry and Physics 45 



incipient nucleation (ionization) of dust-free wet air in the lapse of time. No 

 periodic variations are detected, but the vapor nucleation changes about 2 per cent 

 per degree centigrade. A new series of experiments is then undertaken to standard- 

 ize the coronas of cloudy condensation in dust-free air in terms of the nucleation 

 represented. This is done both by the old method of diffraction from a single point 

 of light and by a new method in which the coronas from two sources are brought 

 into contact. The results obtained are next applied for exhibiting the distribution 

 of ions and vapor nuclei in dust-free wet air in relation to size. Finally, a systematic 

 study is made of the behavior of residual water nuclei, i. e., nuclei of pure water 

 obtained from the evaporation of fog-particles precipitated on vapor nuclei or on 

 ions in dust-free wet air. 



No. 96. (Part II.) BARUS, CARL. Condensation of Vapor as induced by Nuclei 

 and by Ions. Report IV. Octavo, vni+84 pages, 21 text figures. Pub- 

 lished 1910. Price $0.75. 



In the first part of this report the author treats the properties of the nuclei of 

 water vapor, showing among other things that they persist longer in proportion as 

 the evaporation of the parent fog-particles is more rapid. The standardization and 

 efficiency of fog-chambers are then taken up, the results being tested by the coronas 

 of monochromatic light (mercury vapor). These results have a direct bearing on 

 the theory of coronas. Some space is given to the displacement of ions by exhaus- 

 tion, as evidenced by the occurrence of maxima of ionization in the fog-chamber. 

 Finally, the attempt is made to standardize the coronas by the aid of Thomson's elec- 

 tron, the charge of which is known. Independent methods, using either the velocity 

 of the ions or the decay constants, are tested for this purpose. The voltaic poten- 

 tial difference between conductors separated by an ionized medium is investigated. 



No. 149. BARUS, CARL. Production of Elliptic Interferences in Relation to Inter- 

 ferometry. Octavo, vi+77 pages, 33 figs. Published 1911. Price $1.25. 

 Part II. Octavo, pages vi+79-168 figs., 34-64. Pub. 1912. Price $1.00. 

 Part III. Octavo, pages vi+169-273, figs. 65-119. Pub. 1914. Price $1.00. 



In the cases of coronas there is a marked interference phenomenon superposed 

 on the diffractions. This suggested the present investigation, which aims at a sim- 

 plification of the effect in question by bringing two complete component diffraction 

 spectra, from the same source of light, to interfere. Many ways are shown to be 

 available, the methods being either direct (as discussed in Chapters II and III, where 

 a plane mirror immediately behind the grating returns reflected-diffracted and dif- 

 fracted-reflected rays) or indirect, using the devices of Jamin, Michelson, and others 

 (Chapters IV and V). In Chapter I a modification of Rowland's apparatus, suitable 

 for plate gratings, is described. The direct method gives equidistant fringes, duplex 

 in character, but rigorously straight throughout the spectrum, their distances apart 

 and inclination being measurable by ocular micrometry. Lengths and small angles 

 are thus subject to micrometric measurement. In case of the indirect method, the 

 half-silvered plate is replaced by the grating; the fringes are, as a rule, approxi- 

 mately elliptic (confocal) throughout the spectrum, and the fringes again partake of 

 both a drift and a radial motion when the interferometer adjustment is altered. 

 Drift and radial motion may be regulated in any ratio. The theory of the subject 

 is worked out, in so far as it bears on the practical results obtained. 



Part II is in the main a direct continuation of the preceding and refers chiefly 

 to applications of the displacement interferometer. The first section describes cer- 

 tain interferences obtained when two originally coplanar halves of a reflecting 

 grating move normally to the ruled surface, the line of separation being parallel to 

 the rulings. It suggests the occurrence of interference rings when the source of light 

 in the case of coronas is not simple but a doublet of two interfering beams, and that, 

 for layers of fog-particles whose distance apart is commensurate with wave-length, 

 interferences should be superimposed on the coronal diffractions as actually observed. 



Certain subsidiary investigations follow: The effect of the thickness of the cloud 

 layer on the type of corona obtained; on the efficiency, cat. par., of different sizes 

 of fog-chambers; on the rate of decay of different sizes of nuclei, etc. The dis- 

 placement equation adduced in the first report is rigorously tested, a method for 



