SOLAR RESEARCHES BY JANSSEN—PLUVINEL. vay 
manner. He calculated that when the height of the sun above the 
horizon is less than 4 degrees the thickness of the layer of air 
traversed by the solar rays would be sufficient to produce these lines. 
And so he went to the Desert of Sahara in order to observe the sun 
under these conditions and noted the presence of these bands when 
the sun had reached precisely this altitude of 4 degrees. 
This remarkable law must have an importance for the theory of 
molecular physics which has not yet been sufficiently appreciated. 
In his researches at the laboratory of Meudon, Janssen was not con- 
tent with trying the effect of the variation of the length and pressure 
of his column of gas traversed by the light; he also raised the gas to 
high temperatures in order to approach somewhat the conditions as 
they exist in the sun. By electrical means, very remarkable for the 
time when they were devised, Janssen was able to raise his gas to 
a temperature of 900° C. No new phenomenon appeared at that 
temperature, but the visibility of the absorption bands was somewhat 
increased. 
The absorption produced by water vapor also was studied at 
Meudon. Already at the beginning of his spectroscopic studies in 
1867, Janssen had observed the absorption spectrum of water vapor 
by causing the luminous rays to pass through a tube 37 meters long 
filled with this vapor. This remarkable experiment, made at the gas 
manufactory of Vilette, allowed Janssen to recognize the principal 
lines due to the absorption of water vapor. This research was taken 
up under better conditions and with better apparatus at the labora- 
tory at Meudon in 1887. 
The object of this study of the spectrum of water vapor was to find 
out whether it exists in the atmospheres of the planets. This question, 
which is of capital importance to astronomers, always very greatly 
interested Janssen. In 1867 on Mount Etna, and then in 1869 on 
the Himalaya Mountains, Janssen observed the spectrum of Mars to 
see whether he could detect in its spectrum the principal lines due to 
the presence of water vapor. To that end he compared the spectrum 
of Mars and the moon when these two bodies were at the same altitude 
above the horizon. Janssen concluded from his observations that the 
spectrum of Mars gave plain evidence of the presence of the vapor of 
water in the atmosphere of that planet and he considered this evi- 
dence sufficiently decisive to maintain these conclusions when, in 1895, 
Campbell announced that the great instruments of the Lick Observa- 
tory would not show to him the trace of any water vapor on Mars. 
And now, very recently, Mr. Slipher, of the Lowell Observatory, has 
obtained photographs in which the lines due to water vapor appear 
more intense in the spectrum of Mars than in that of the moon. This 
seems to support Janssen’s conclusions. 
45745°—sm 1909——17 
