SCIENTIFIC SUMMARY. 
327 
resonator, they cause the serrations of its flame successively to coincide with 
and to bisect those of the organ-pipe flame. Students, he says (“ Silliman’s 
American Journal after such exhibitions do not depart from the room 
with their usual scepticism as to the existence of an acoustic wave-length, 
but look upon the tubes as measures of actual entities. 
The Blue Colour of Sky ; its Cause, M. Colas (in Les Mondes ”) first 
refers to the blue colour exhibited by the pure water of certain lakes, and 
says that it is due to the therein dissolved or very minutely divided gela- 
tinous silica, quoting as instances the lake of Geneva, the water of the 
d’Huis, and more particularly the water of a source near to the Mont-Dore 
(Departement du Puy-de-D6me), which water is even bluish-coloured when 
placed in a white glass decanter. The blue colour of the sky is referred by 
the author to the same cause, viz., very finely divided gelatinous silica 
(hydrated silica) kept in suspension in the clouds on account of its great 
lightness. 
Spectrum of the Aurora Borealis. — An attempt has been made by Herr 
Vogel to identify the spectrum of the aurora with that of air, and he has 
arrived at results which, if not absolutely conclusive, render the identifica- 
tion at least probable. The author employed a direct vision spectroscope 
with five prisms, collimator and observing telescope, which last by means of 
a micrometer screw could be moved so as to bring different parts of the 
spectrum into the centre of the field of view. The distances between the 
spectral lines could be read ofi* in fractions of a revolution of this screw. 
By repeated measurements of about 100 lines, aided by Angstrom’s atlas, 
the micrometer readings could be converted directly into wave-lengths. 
With this instrument Vogel determined the wave-lengths of 7 lines with 
the following results : 
Wave-length 
Probable error 
Remarks 
00006297 
0*0000014 
Very bright band. 
0*0005569 
0*0000002 
Brightest line. 
0*0005390 
Very faint line. 
0*0005233 
0*0000004 
Pretty bright. 
0*0005189 
0*0000009 
Bright where the red line appears ; 
otherwise faint. 
0*0005004 
0*0000003 
Very bright. 
0*0004694] 
Broad band somewhat less bright 
0*0004663 1 
0*0000003 
in the. centre ; very faint where 
0*0004629 J the red line appears. 
Pogg. Annal. CXLVI. 569. 
The Relation between Heat and Electricity. — This subject has received 
discussion from Mr. F. Guthrie in a paper which he presented to the 
Eoyal Society, and which is published in an early number of the Che- 
mical News.” The author says that it is found that the reaction between 
an electrified body and a neighbouring neutral one, whereby the electricity 
in the neutral body is inductively decomposed and attraction produced, 
undergoes a modification when the neutral body is considerably heated. 
Under many circumstances it is found that the electrified body is rapidly 
and completely discharged. The action of discharge is shown to depend 
