152 MR. W. CROOKES OK THE ILLUMINATION OF LINES OF 
The cessation of phosphorescence by the heating of the phosphorescent body was 
observed by Professor Stokes. In his paper on the “ Change of the Refrangibility 
of Light,” in the Phil. Trans, for 1852, at page 532, it is mentioned that glasses and 
certain sulphides, which were phosphorescent when cold, lost their sensitiveness on 
heating, but recovered it again on cooling. 
MECHANICAL ACTION OF PROJECTED MOLECULES. 
541. It was noticed that when the coil was first turned on, the thin glass film was 
driven back at the moment of becoming phosphorescent. Tins seemed to point to an 
actual material blow being given by the molecular impact, and the following experi¬ 
ment was devised to render this mechanical action more evident. 
A large somewhat egg-shaped bulb (fig. 15, elevation) is furnished at each end with 
fiat aluminium poles, a and b ; a pointed aluminium pole is inserted at c. At d, a little 
indicator is suspended from jointed glass fibres, so as to admit of being brought into 
Fig. 15. 
any position near the middle of the bulb, by tilting the apparatus. The indicator 
consists of a small radiometer fly 8 millims. in diameter, furnished with clear mica vanes 
2 millims. across, and delicately supported on a glass cup and needle point. A screen? 
cut out of a flat aluminium plate 12 millims. wide and 30 millims. high, is supported 
upright in the bulb at e, a little on one side of its axis, being attached to the bulb by 
a platinum wire passing through the glass, so that if needed the screen e can be used 
as a pole. 
542. This apparatus was designed with a double object. The indicator fly is not 
blacked on one side or favourably presented, therefore if immersed in a full stream of 
projected molecules, there will be no tendency for it to turn one way rather than the 
