266 Frederick Guthrie on Salt Solutions 



one of Browning's larger direct-vision spectroscopes fitted in this 

 way. Large aperture, however, both of prisms and lenses, is 

 almost indispensable in observations of faint auroral spectra. I 

 close this by adding a few remarks to Professor HerschePs list of 

 auroral lines (appended to his letter), which may be useful to 

 other observers. 



Remarks by way of addition to Professor HerschePs list of 

 auroral lines*. 



1. Red line.— On my scale coincident with telluric group a 

 in solar spectrum. I found at the same time a faint telluric 

 band or group coincident with the yellow-green line. 

 ; 2. Yellow- or citron-green line. — I always see this more pale 

 green than yellow ; sometimes nickering and changing in in- 

 tensity (Professor Herschel has also noted this on one occasion). 

 Not always sharp. Procter has recorded it nebulous ; might not 

 its appearance in regard to width, intensity, and sharpness be 

 made a test of height of the aurora? 



3. Greenish-blue or blue lines. — One cannot help suspecting 

 difference of position as well as of intensity of lines in the central 

 portions of auroras. We know how varying aurora? are in general 

 character and appearance; and although no doubt much must be 

 allowed for hasty observation, often made with imperfect instru- 

 ments so far as measuring is concerned, there still seems a resi- 

 duum of considerable difference in the position of lines to be 

 accounted for. Lines Nos. 1 and 2 are closely fixed; but I 

 would counsel special attention to the places of the fainter lines ; 

 and it might be useful to put No. 2 out of the field, or hide it 

 by a bar while observing these. 



Indigo and violet lines, Nos. 5 and 6. — As mentioned before, 

 No. 5 seems most indefinitely positioned ; No 6 I have never 

 been fortunate enough to see. 



XXX. On Salt Solutions and Attached Water. 

 By Frederick Guthrie. 



II. 



[Concluded from p. 218.] 

 Chlorides of the Alkaline Earths. 



§ 78. (JHLORIDE of Barium.— The chloride of barium in 

 saturated solution becomes a solid cryohydrate at 

 — 8° C. Of the portion last to solidify, 6*6790 grms. gave, on 

 evaporation and- heating to 350° C, 1-5490 Ba Cl 2 . This cor- 

 responds to 23*2 per cent. Of a previous crop of crystals, 6*1350 



* See also 'English Mechanic/ January 16th, 1874, No. 460. 



