124: NOTES. 
not of negative, knowledge, —a conclusion reached from what 
see, and not from what we fail to see. l 
Purr Warer. — Dr. Burdon Sanderson, F.R.S., found it 
sible to obtain optically pure water. The fusion of ice fi 
the nearest approach to this standard. , 
Ramway Dust. — The “Manchester Guardian” publis 
study of railway dust made by Mr. J. Sidebotham, who finds 
consist, in the case examined, about one-half of particles of u 
and the other half of cinders, sand, ete. Some of the parti : 
iron were magnetic, and most were sharp, rough and irritati 
NOTES. 
We make the following extracts from a letter to one a the 
‘tors from Mr. Dall, Chief of the Coast Survey Expedition 
plore the hydrography and natural history of Alaska. It is @ 
Harbor of Hiuliuk, Unalashka, Alaska Terr., Oct. 30, 1871: 
‘t We arrived here on the 23d of September after a dis p : 
passage of twenty-six days from San Francisco, during ; 
however, we obtained some very interesting observations m 
WwW inc 
spring to have it approximately complete. Tidal ant | 
observations are going on, we have taken many hundr 
gles and shall go to sounding bye and bye. 4 
e island when we came was a mass of verdure up to™ 
caps of the highest peaks. There are no trees, excepi 
than six feet high and an inch or two thick. I went on ag 
T A ax : ENER he island and with 
ing trip the other day in the interior of the 1 packs for kini 
