﻿236 Intelligence and Miscellaneous Articles. 



creased, the sodium-line is added to the spectrum of the gas, as it 

 is seen that while using the induction-current the capillary part 

 of the spectrum-tube in front of the slit is heated. With a suitable 

 striking-distance, the sodium-line is so bright that it far exceeds in 

 intensity the lines of the gas-spectrum if a hydrogen-spectrum tube 

 has been taken. If the striking-distance be slightly increased, the 

 bright lines of the calcium- spectrum occur with a beauty and preci- 

 sion which can scarcely be obtained in any other way. Passing be- 

 yond the striking-distance which has furnished this spectrum, the 

 entire phenomenon changes. The luminous line in the spectrum- 

 tube is of such dazzling brightness, that even looked at by daylight 

 it leaves a long-continued after-image in the eye. This luminous 

 line, looked at in the spectrometer, shows a considerably bright con- 

 tinuous spectrum, in which, however, the position of the sodium-line 

 appears quite dark ; we thus obtain an artificial spectrum with one 

 dark line, or, since the mode of formation is the same, with one Fraun- 

 hofer's line. 



That this line is formed in the same manner as, according to 

 KirchhofF, Fraunhofer's are in the solar spectrum, is at once seen on 

 looking at the tube after the experiment ; the inside of the capillary 

 tube is seen to be very strongly corroded by detached glass splinters, 

 so that after the experiment has been often made the glass has be- 

 come quite dull. These glass particles, which each discharge de- 

 taches, are at the same time raised to incandescence, and the light 

 of these ignited solid particles furnishes the bright continuous spec- 

 trum. These solid particles glow, however, in an atmosphere of 

 sodium-vapour, and this holds back the same light which was ob- 

 served before the solid particles were detached; hence where this 

 light ought to be in the spectrum there is a dark part. There is here 

 seen in its individual stages the formation of Fraunhofer's line ; for the 

 discharge of the Leyden jar first produces the ignited atmosphere, and 

 then the far more brightly glowing core in it. With the formation 

 of the glowing solid core the bright line furnished by the atmosphere 

 becomes obscure. 



It might be expected that, besides the scdium-line, the calcium- 

 line and that of the gas would appear dark. I have been unable 

 to perceive these lines ; the reason doubtless is that calcium-vapour 

 is not sufficiently dense to allow the absorption of the light corre- 

 sponding to it to be so strong that the inversion can occur. — Poggen- 

 dorfF's Annalen, No. 9, 1868. 



ON THE QUANTITY OF ELECTRICITY PRODUCED BY THE ELEC- 

 TROPHORUS MACHINE EXPRESSED IN ABSOLUTE MEASURE. 

 BY F. KOHLRAUSCH. 



The working of Holtz's electrophorus machine has been investi- 

 gated by means of the unit-jar, and particularly as compared with 

 that of the friction machine. An absolute measurement in this way 

 would be attended with the greatest difficulties. If we restrict our- 



