000 Scientifie Proceedings, Royal Dublin Society. 
found in different localities ; for one of the objects of this work 
was to ascertain whether meteoric dust descended upon the Earth, 
and this could only be done by ascertaining whether dust from 
terrestrial sources resembled in composition that known to be of 
meteoric origin. 
The method of examination was by spectrographic analysis, 
which is of extreme delicacy, and practically the only means 
available for investigating the composition of complex substances, 
with very minute quantities of material, such as could be obtained 
from hail-stones and drops of rain. The largest quantity of 
material derived from such a source was 0:08 gr. The spectra 
photographed were obtained by burning in the oxyhydrogen 
flame the substance wrapped up in, or distributed over, the surface 
of an ashless filter-paper. From the number of lines observed 
belonging to the different elements which can be photographed in 
this way, also from their relative intensities, the proportions of the 
different substances present could be ascertained, and the different 
spectra could be compared. It thus became evident that different 
kinds of dust could be broadly classified according to their 
composition, and their origin ascertained. It may be well to 
remark, at this point, that arsenic and antimony cannot be 
detected by this method when present in minute proportions, and 
that a chemical method, such as Marsh’s test, is more sensitive 
than either the spark or flame-spectrum of arsenic. . 
There are two samples of dust which, on inspection, appeared 
to be of an unusual character, to which attention may be parti- 
cularly directed. 
(I.) Solid matter which fell in or with hail in a hail-storm on 
Wednesday, April 14th, 1897, and was collected by Professor 
O’Reilly at a window facing the large open space of Stephen’s 
Green, at the Royal College of Science, Dublin. It contained 
iron, sodium, lead, copper, silver, calcium, potassium, nickel, 
manganese (a trace) ; gallium and cobalt gave doubtful indications. 
(II.)} Solid matter from hail and sleet collected by Professor 
O’Reilly on plates upon a window-sill of the Royal College of 
Science, Dublin, during a very heavy shower, from 2.30 till 
3 o'clock, in the afternoon of March 28th, 1896. 
Total weight of the dust 0°1018 gramme, of which 0:08 
gramme was burnt in the oxyhydrogen flame. The colour of the 
