SIR W. CROOKES ON THE PHOTOGRAPHIC SPECTRA OF METEORITES. 
417 
tube ceased to conduct the current. The aerolite was then heated so as to expel 
the occluded gases, the pump set going, and the spectrum of the gas examined at 
various pressures down to about 20 mm.—when the pumping was stopped. 
The calcium tube B was now strongly heated with a “ Meker ” burner ; at first the 
gauge was depressed. owing to gas liberated by the heat, and then absorption 
suddenly took place, and the gauge ran up to one or two millimetres or less. The 
spectrum was kept under examination the whole time, and for convenience the 
spectrum given by helium was thrown on to the eye-piece by means of a right-angled 
prism, so that any indication of D 3 , however faint, could be instantly observed. 
Working in this way no indication whatever was found of the lines of any of the 
inert gases. The absorption in each case could be carried until the tube refused to 
conduct the current. The final lines observed before the tube ceased to be luminous 
were those of mercury—generally accompanied by a faint trace of the red line of 
hydrogen. 
In each aerolite examination—-after having satisfied myself of the absence ol 
unabsorbable gases—a further quantity of gas was driven out by heat and the 
pressure adjusted to about 2 mm., the vacuum tube being sealed off at this pressure 
and kept with full particulars of its source for future reference. 
The amount of gas that could lie driven out of the aerolites by heat varied 
considerably. In some cases it was possible to expel several successive volumes of 
gas and each time to repeat the absorption by calcium ; in other cases the amount 
