PRESIDENT’S ADDRESS. 43 
They found by means of the X rays that the otherwise per- 
fectly opaque alloy had an internal structure similar to that 
of a saline solution from which crystals had separated, the 
details being quite visible to the naked eye. They also 
hope to obtain similar results with aluminium alloys, since 
it is a metal also transparent to the X rays. 
It is said that glass bulbs tinged with didymium chloride 
give ared instead of a green phosphorescence, and yield rays 
of twice the intensity of those from Crooke’s tubes made of 
ordinary glass; hence the chemical composition of the glass 
is a factor in their formation or transmission. 
There appears to be avery close relationship between the 
atomic weights of the elements and their permeability to 
the X rays. 
Lithium, with an atomic weight of 7,is more transparent 
to them than sodium with an atomic weight of 23, and 
sodium is in turn more permeable to them than potassium 
with an atomic weight of 39. It has been found that a 
plate of lithium must be ten times as thick to produce the 
same amount of absorption as a plate of sodium. Hence 
the chemist may use this property in determining doubtful 
atomic weights. 
C. Doelter finds that phenacite (beryllium silicate) is 
always perfectly transparent; and that olivine, zoisite, and 
sphene are, like calcite, almost opaque, and vesuvianite is 
only slightly less so. 
Diopside, hiddenite, and topaz are semi-transparent. The 
diamond, sapphire, and ruby, both natural and artificial, 
are almost transparent, but paste imitations are opaque, and 
they are accordingly easily recognised by means of these 
rays. In fact,all gems, except garnets and zircons, contain 
elements of low atomic weight, and they are, therefore, 
more or less transparent to the X rays, which thus afford 
assistance in recognising them. 
Doelter also finds that minerals with a greater density 
than 5 seem to be opaque to the rays. 
