338 Dana on Nitrous Gas. 



Jhis halo intersecting the second on the east and west sides, 

 but at that time the intersections on the north side were 

 not visible. These appearances were not of long continu- 

 ance, nor constant in their arrangement, but, beautiful and 

 sublime beyond description. Unwilling to trust to memo- 

 ry as the only memento of such extraordinary phenomena 

 I made a sketch at the time, which, imperfect as it is, is 

 probably the only one taken, and is now respectfully pre- 

 sented you. I regret that it did not h\\ to the lot of some 

 person better qualified than myself to describe it. (See 

 plate 3. Fig. 4.) 



Of halos, having the sun'in the centre, perhaps the cause 

 has been satisfactorily explained, but of those whose peri- 

 phery is in the sun, as respects their locality I can myself 

 form no idea. From what was seen at that time, 1 am how- 

 ever fully satisfied there is a possibility of the atmosphere 

 being so chai'ged, as that the whole concave of Heaven 

 wouid be covered with similar appearances. T. K. 



Art. XVII. — On the Theory of the action of the Deutoxide- 

 of Azote, or Nitrous Gas in Eudiomelry ; by James 

 Freeman Dana, Professor of Chemistry and Mineralogy 

 in Dartmouth College, 



The deutoxide of azote or nitrous gas appears to be the 

 first substance introduced for the express purpose of ana- 

 lysing the atmosphere. It owes its application, in the pro- 

 cesses of Eudiometry, to Priestley, ils discoverer ; its use 

 is founded on the facility of its union with oxygene gas \ 

 when the two gases come in contact, red vapours are pro- 

 duced, which are rapidly absorbed by water. 



It appears to be established, by multiplied experiments, 

 that the atmosphere contains ;21 volumes of oxygene gas in 

 100; yet the deutoxide of azote has sometimes indicated 

 much more, at other times much less, and has seldom af- 

 forded uniform results. Priestley was led to suppose that 

 there was a very perceptible difference, as indicated by 

 nitrous gas, between the external air, and the air of his 

 study after a number of persons had been sitting there ; and 

 that air in the neighborhood of York was not as good as 

 that near Leeds. Yet there is not found to be a percepti- 



