732 
through evaporation and precipitation and 
by means of electricity or some more useful 
force. 
By a general comprehension of the prin- 
ciples of nutrition, food will be more whole- 
some and more potent. The general ac- 
ceptance of the principles of hygiene will 
make the average life of man longer and 
his usefulness more fruitful. Man will not 
only live longer, but he will be happier and 
practically free from the threats of enzymic, 
contagious and epidemic diseases. When 
this Society meets on that founders’ day, 
the membership will be nearly 10,000 and 
its organization will reach to all quarters of 
our imperial country. The number of 
those who to-day are members and who 
shall live to 1976 is not large, possibly nil, 
but many who are infants to-day will be 
the revered old men on that centennial oc- 
easion. The orator who will address you 
on that day is perhaps not yet born. Ihope 
he will take for his theme, the ‘ relation of 
chemical work to the advancement of man- 
kind in the past century.’ He will find in 
the development of some of the thoughts 
which I have tried to bring to your atten- 
tion to-night the most potent causes that 
make for the good ofman. Insuch a light 
as he can shed on life and its conditions 
the coming man will be able to see the true 
dignity of chemistry. H. W. WIitey. 
U. S. DEPARTMENT OF AGRICULTURE. 
DIAMOND-GLASS FLUORESCENCE. 
Some five years ago I had occasion to cut 
a large number of photographic dry plates 
to smaller sizes. They were cut in the 
usual way—with a diamond and on the 
side of the plate opposite the film. In 
developing it was noticed that the film, toa 
breadth of a few millimeters along the edge 
of the plate, turned dark as if exposed to 
light. 
Several possible explanations suggested 
themselves. 
SCIENCE. 
(N.S. Von. XIII. No. 332. 
1. The breaking of the glass might pro- 
duce momentary fluorescence and a fogging 
of the film near the break. 
2. The breaking or tearing of the film 
might result in some sort of change in its 
character. 
3. The scratching of the diamond might 
set up mechanical disturbances or vibra- 
tions in the glass and these might affect the 
film. 
4. The frietion between the diamond and 
the glass might cause a momentary fluores- 
cence along the line traced by the diamond, 
and the radiation might penetrate the glass 
and fog the film on the other side. 
The first and second suggested explana- 
tions were thrown aside at once, for the 
dark band in the film was found along the 
diamond scratch, whether the plate was 
broken or not. 
That the third is not the true explanation 
was shown in several ways. The breadth 
and density of the dark band did not appear 
to depend upon the depth of the cut or the 
rapidity with which it was made. The line 
was always of about the same breadth on 
the same plate, but of different breadths on 
different plates. Moreover, the film always 
developed first on the side next the glass. 
The effect was noticeable on the most sensi- 
tive plates only. 
Fe9t- 
Let f represent the film on a section of 
the glass plate g, perpendicular to the dia- 
mond scratch s. Let s bea source of radia- 
tion. 
All rays (as sc) outside the critical angle 
i are totally reflected and hence do not 
affect the film. Those having an incident 
angle smaller than i penetrate the film and 
