616 BUNSEN MEMORIAL LECTURE. 



Breslau. that Bunsen turned his jittontion to iisiiio- the l)atterv for elec- 

 trolytic preparation of inetals, some of which had not hovn ()l)tuined 

 in a coherent condition, and others had only l)een prepared in such 

 minute quantities that their physical and chemical properties could not 

 be properly studied. The tirst of these metals he attacked was magne- 

 sium (Annalen, 1852 (32), 137), the reduction of which had vainly been 

 attempted by Davy, and oidy with very partial success by Bussy in 

 1S80, The difficulty which had hitherto stood in the way was the 

 fact that the globules of molten magnesium are lighter than the fused 

 magnesium chloride used as the electrolyte, and that on their forma- 

 tion they rise to the surface and burn. To avoid this Bunsen adopted 

 the ingenious plan of cutting the carbon pole, on which the metal 

 forms, into pockets, inside which the magnesium is deposited, and 

 from which the molten globule can not escape. By means of the tan- 

 gent galvanometer Bunsen measured the absolute units the electricity 

 employed, finding that the quantity of magnesium reduced is 2.45 

 grams, while the theoretical yield of metal is 4.()9H grams. Having 

 obtained the metal in some quantity, he determined its physical and 

 chemical properties, showed how it could be pressed out into wire, 

 and measured the luminous intensity of the burning metal. This he 

 found to be 500 times that of a candle Hame. 



Some seven years later, he and I measured the actinic value of the 

 light emitted l)y l)urning magnesium, and showed that it could be 

 used for photographic })urposes. We found that a burning surface of 

 magnesium wire, which, seen from a point of the .sea's level, has an 

 apparent magnitude ('(jual to that of the sun. effects at that point the 

 same chemical action as the sun would do if shining fi'om a cloudless 

 sk}" at the height of 9° 53' above the horiztm. On comparing the visi- 

 ble brightness of these two sources of light, it was found that the 

 brightness of the sun's disk, as measured l)v the eye, is 524.7 times as 

 great as that of burning magnesium when the sun's zenith distance is 

 67^ 22', while at the same zenith distance the sun's chemical brightness 

 is only 36.6 times as great. Hence the value of this light as a source 

 of chemically active rays for photographic purposes is at once appar- 

 ent. The application of magnesium as a source of light has become 

 of technical importance, A burning magnesium wire of the thickness 

 of 0.297 millimeter evolves as much light as 74 stearin candles of which 

 five go to the pound. If this light lasted one minute, 0.987 meter of 

 wire, weighing 0.120 gram, would be burnt. In order to produce a 

 light equal to 74 candles burning for ten hours, whereby 20 pounds of 

 stearin is consumed. 72.2 grams or 2^ ounces of magnesium would be 

 needed. The light from burning magnesium has been emplo3'ed for 

 signaling, and for military and naval purposes, and it is especially 

 used in pyrotechny. 



Perhaps the most interesting of these applications of the battery is 

 that of the preparation of the metals of the alkaline earths (Journ. 



