2 ' 
splashed on the surface, will serve to drive back the scum 
for a certain distance. This may be shown by scattering 
some flowers of sulphur on the surface. This powder is 
insoluble and produces no scum, and hence it serves ad- 
mirably to show the motion of the surface and whatever 
scum there may be upon it. If when the surface is so dusted 
a splash be made by a stick so as to throw drops on to the 
sulphured surface, at the first splash no floating drops are 
produced ; but after two or three splashes in rapid succession 
it will be seen that the sulphured scum has been driven 
back by the falling water, leaving a patch of clear surface, 
and on this drops will float in large numbers and of all 
sizes. These drops are entirely confined to that portion of 
the surface which is clear. The drops, either by their initial 
motion or by the current of air, glide rapidly over the surface 
from the point at which they are formed. When, however, 
they reach the edge of the scum they disappear, apparently 
somewhat gradually. I have this summer made the ex- 
periment on several ponds and on various days, and I have 
never found any difference. Any scum, however trans- 
parent, prevented the drops, and they always floated in large 
numbers when the scum was driven back in the manner 
described, by the wind or any other way. 
This result points to the conclusion that whatever may 
be the cause of this suspension, it depends only on the sur- 
face of the water being pure, and not at all on the tempera- 
ture or condition of the air. 
“ On the Mean Intensity of Light that has passed through 
Absorbing Media,” by James Bottomley, D.Sc., F.C.S. 
In colorimetric experiments the areas compared are as- 
sumed to be of the same tint throughout. When, however, 
we look through columns of coloured liquid at external 
white surfaces, this condition will not be exactly fulfilled. 
