300 Intelligence and Miscellaneous Articles. 



and we find, for the mean intensity I, 



2E 



1=^= * . . . . 



J. — 



(»+*?7 



The electrometer, in the conditions in which I use it, does not 

 give this mean intensity, but the square root of the mean of the 

 squares of the intensities — that is to say, an intensity I' satisfying 

 the condition 



T /»^0T+! A 2 T 



r»i =A* \ sm\mt-27rd>)dt = ~^. 



2 Jt=$T 4 



From this we deduce 



and consequently 



I 2*/2 

 The formula to be compared with the experiments is therefore 



V2 



r= 



B"^) 1 



— that is to say, the formula to which I had been empirically con- 

 ducted. — Comjptes Rendus de VAcademie des Sciences, Sept. 6, 1880, 

 t. xci. pp. 468-470. 



ON AN ACOUSTIC METHOD OF DETERMINING VAPOUR-DENSITIES. 

 BY H. GOLDSCHMIDT*. 



From Laplace's formula for the velocity of sound in gases there 

 results for the ratio of the densities of two gases d and D which, 

 successively set in vibration in the same tube, give tones of the 

 vibration-periods n and N, 



d : D = N 2 : n\ 



For D = l (air), 



d=W : n 2 = air-tone 2 : gas-tone*. 



The author raps the test-tube filled with the gas in question, and 

 seeks the resulting tone upon a violin. This procedure is applicable 

 also to substances which are liquid at ordinary temperature. The 

 test-tube, containing a small quantity of the substance, is closed 

 above with a caoutchouc stopper through which passes a capillary 

 tube, and brought to evaporation in steam. When no more vapour 

 issues from the capillary tube, the stopper is pulled out and the 

 tone then heard is determined. The observed agree very well with 

 the calculated values. — Wiedemann's Beibldtter, 1880, No. 7, p. 500. 

 * Chem. Ber. xiii. pp. 763-771 (1880). 



