ELECTRICITY. 



245 



salt. Thus water containing one one-thou- 

 sandth part of its weight of chloride of strontium 

 gives very distinctly the orange and blue rays 

 characteristic of strontium. With concen- 

 trated solutions the effects are more marked, and 

 with the chlorides in particular they are very 

 brilliant. Thus the chlorides of strontium, cal- 

 cium, sodium, magnesium, copper, and zinc, give 

 fine effects. But other substances, such as vari- 

 ous compounds of barium, pbtassium, antimony, 

 iron, manganese, silver, uranium, etc., give ef- 

 fects which are more or less marked. In gen- 

 eral the lines are more numerous than in the 

 spectra of flames containing the same saline 

 elements, which doubtless arises from the 

 higher temperature, but in all cases the lines 

 are the same as those given by Bunsen and 

 Kirchhoff. Thus with a saturated solution of 

 chloride of strontium, besides the orange and 

 the clear blue, we see two violet rays, one more 

 intense than the other, several green rays, one 

 of which is particularly distinct, and a certain 

 number of feebler rays in different parts of the 

 spectrum. Chloride of lithium, besides the red 

 and the feeble orange, gives a very vivid blue 

 ray. A concentrated solution of chloride of 

 calcium gives a great number of rays, among 

 which the dark-blueray is very intense. Chlo- 

 ride of magnesium, besides other lines, gives two 

 very bright -green and one clear -blue ray. 

 Chloride of zinc gives a red ray, three brilliant 

 blue rays, and a very intense violet line. Ni- 

 trate of silver gives, among other rays, two of a 

 vivid green. In conclusion, the author points 

 out the very obvious and marked advantages 

 which this method of observation offers in cer- 

 tain cases over the usual method of ignition in 

 a non-luminous flame. 



The Electric Spark in a Vacuum. MM. 

 Alvergniat, Freres, have invented an apparatus 

 to demonstrate that the electric spark cannot 

 pass through a perfect vacuum. They create a 

 nearly absolute vacuum, by means of a mercu- 

 rial pneumatic machine, in the tube that serves 

 for the experiment. This contains two plati- 

 num wires, placed at a distance of two milli- 

 metres from each other. The tube is heated 

 to dull redness, and, when that point is attained, 

 the process of making the vacuum is still con- 

 tinued, and the electric spark passed until it 

 ceases to be transmitted through the interior of 

 the tube The tube is then hermetically sealed 

 and separated from the machine. In a tube thus 

 prepared, notwithstanding the slight distance 

 between the two platinum points (two milli- 

 metres), electricity absolutely ceases to pass. 



Magnetism and Molecular Changes. Exper- 

 iments made by M. Treves, a French naval 

 officer, prove that a steel bar, magnetized by 

 an electric current, undergoes some molecular 

 change while magnetized. Two exactly iden- 

 tical steel diapasons, giving sounds precisely in 

 unison, were selected. A small mirror was so 

 placed in relation to each, that, when vibrations 

 were^ struck upon either diapason, a figure of 

 the vibrations was reflected into the mirror 



the figures from the two diapasons being pre- 

 cisely similar, in consequence of the metals be- 

 ing in unison. Round one diapason was then 

 placed a powerful bobbin of wire, actuated at 

 will by a current from eight Bunsen's elements. 

 The other diapason was left unchanged. Im- 

 mediately on passing the current through the 

 bobbin, exciting the diapason, and rendering it 

 magnetic, the following changes were noticed 

 in the figure reflected from it in the mirror : 

 The luminous circle that had previously been 

 constant was observed to alter immediately 

 from itself into an ellipse, and oscillate from 

 right to left with a speed that enabled the new 

 vibratory movement to be measured. This 

 speed was faster or slower in proportion to the 

 increase or diminution of the number of ele- 

 ments used. Whenever the current was shut 

 off, the normal state of the diapason returned, 

 and the fixed luminous circle, due to its natu- 

 ral condition when vibrating, reappeared. M. 

 Troves conducted similar experiments upon 

 diapasons of soft iron, and of steel of various 

 sizes, arriving at the similar results. M. Faye, 

 in a note to the French Academy, accompany- 

 ing a report of the facts, says that the new ex- 

 perimental method of M. Treves has made a 

 marked step in the science of magnetism. 



The Aurora Borealis as a Weather Prognos- 

 tic. Mr. Murray Gladstone, of England, has 

 for many years studied the aurora borealis as a 

 weather prognostic. He has observed that, 

 when the coruscations are vivid, and particu- 

 larly if they extend toward the zenith, or show 

 much motion, they are almost invariably fol- 

 lowed by a gale of wind with rain from S.W., 

 within from forty-eight hours to four days. 

 The more brilliant and lively the appearance 

 and motion of the aurora, the earlier the gale 

 which follows takes place, and the greater is 

 its severity. Slighter manifestations of the 

 northern lights are not followed by any ap- 

 preciable changes of weather. In explanation 

 of the connection between the two sets of phe- 

 nomena, Mr. Gladstone suggests that when a 

 larger body than usual of light air from the 

 south begins to descend upon the cold stream 

 of air coming from the north, as those opposite 

 currents in the atmosphere come into close 

 proximity, their negative and positive electri- 

 cities produce coruscations. The rarity of the 

 atmosphere and the great elevation probably 

 prevent (at least for the most part) any sound 

 or thunder being heard ; and the former cause, 

 joined with the manner in which the currents 

 approach each other, may probably occasion 

 the shooting, flickering movements of the 

 aurora. The arches of boreal light frequently 

 seen stretching from E. to W. may be pro- 

 duced by large masses of air charged with 

 opposite electricities meeting each other and 

 feeding the flames quietly and continuously, on 

 an extended front; while the movements of 

 light occasionally occurring throughout the 

 length of these arches may arise from the 

 masses of vapor coming more actively into 



