NATURAL PHILOSOPHY. 145 



BREADTH OF SPECTRAL BANDS. 



Lippich has attempted the application of the dynamical theory 

 of gases and vapors to the explanation of the breadth of the bands 

 of gaseous spectra. The assumption made is that " if it be neces- 

 sary to consider a molecule as a system capable of vibration, the 

 spectrum of an ideal gas in which the molecules could be per- 

 fectly free elastic systems could consist only of a number of differ- 

 ently colored bands of homogeneous light if the vibratory motions 

 of the molecules alone be taken into account." But as, according 

 to the theory of Clausius, the molecules have also very rapid pro- 

 gressive motions, the refrangibility of the rays produced will 

 depend upon the combination of the vibratory and progressive 

 velocities, thus showing the dependence of the breadth of the 

 bands upon the temperature and density of the ignited gas. The 

 author gives a law regarding the ratio of the mean and extreme 

 wave-lengths of the bright bands and dark lines of the same and 

 different gases. 



The comparison of the relative breadth of bands may lead 

 to some important conclusions. The proximity of bands of 

 different breadths in anj 7 spectrum may indicate the presence of 

 gases of different densities or of the same gas in different allo- 

 tropic states. For example, the faint blue lines in the oxygen 

 spectrum may indicate the presence of the denser ozone. The 

 breadth of the spectral bands in the same gas will permit a con- 

 clusion as to the temperature, a fact useful in stellar spectro- 

 scopy, there being noticeable differences in the breadth of the 

 hydrogen bands. The author remarks finally that his conclu- 

 sions apply only to perfect gases, and changes from these to 

 vapors will be noticed by changes in their spectra. 



INTENSITY OF ACTINISM AT DIFFERENT ALTITUDES. 



A paper has been presanted to the Royal Society by Messrs. 

 Roscoe and Thorpe, giving the results of a series of determina- 

 tions of the chemical intensity of total daylight under different 

 altitudes of the sun. The experiments were made on a flat table- 

 land near Lisbon. The method used was founded on the exact 

 estimation of the tint which standard sensitive paper assumes 

 when exposed for a given time to the action of daylight. The 

 experiments were made as follows : 



1. The chemical action of daylight was observed in the ordi- 

 nary manner. 



2. The chemical action of the diffused daylight was then 

 observed by throwing on to the exposed paper the shadow of a 

 small blackened brass ball placed at such a distance that its 

 apparent diameter, seen from the position of the paper, was 

 slightly larger than that of the sun's disc. 



3. Observation No. 1 was repeated. 



4. Observation No. 2 was repeated. 



The mean of observations 1, 2, 3 and 4 was then taken. 



