PHYSICS. 505 



ever publicly exhibited was shown in operation at a meeting of the 

 Literary and Philosophical Society of Manchester, in February, 1879." 

 On tlie question of cost, he claims 200 candles per horse-power; but 

 calling- it 150, we have for the 50 horse-power obtainable from 1 cwt. of 

 coal, 7,500 candles, as against 3,000 from an equivalent value of gas. 

 {N'ature, August, xxvi, p. 356.) 



The experimental committee of the jury of the Paris Electrical Ex- 

 hibition, of which Tresca was president, appointed the following sub- 

 committee to make the tests on incandescent lamps : Barker (chairman), 

 Crookes, Hagenbach, Kundt, and Mascart. They examined the Edison, 

 the Swan, the Lane Fox, and the Maxim lamps, the only four exhibited, 

 The following are their results: At 16 candles, the candles produced 

 per horse-power of current were, for the Edison lamp, 196.4; for the 

 Swan, 177.9; for*the Lane Fox, 173.6; and for the Maxim, 151.3. At 

 32 caudles the Edison lamp gave 307.25 caudles per horse-power of 

 current; the Swan, 202.49; the Lane Fox, 276.89; and the Maxim, 

 239.41. Subsequently a committee, consisting of Tresca, AUard, Le 

 Blanc, Joubert, and Potier, made tests of these lami)s as collateral to 

 their tests of machines. Four Swan lamps, six Lane Fox lamps, one 

 hundred Maxim lamps, and five hundred and twenty eight Edison lamps 

 were tested in the same circuit, the following results being the mean of 

 all these lamps. The results of the tests made by the first committee 

 refer to the light emitted in a horizontal plane, the direction being 45° to 

 the plane of the loop. Those of the second are expressed in terms of 

 mean spherical intensity calculated from the measurements. For com- 

 parison, therefore, the results of both measurements are given below, 

 the mean spherical intensities being calculated for the former measure- 

 ments, the first and second referring to the two committees : 



Maxim. EdisoD. Lane Fox. Swan. 



l8t 2d. Idt. 2d. l.st. 2d. Ist. 2d. 



a. b. 



Mean spherical intensity.. 1.25 2.80 1.36 1.57 1.16 1.64 1.16 2.32 2.19 



Candles per H.P. of current 118.0 151.0 145.3 172.0 119.8 130.6 122.7 211.1 204.7 



"Although the two sets of experiments were made with a different 

 object, and by methods entirely different, it will be observed that the 

 figures of the one approach closely those of the other, thus characteriz- 

 ing very clearly each of the four systems of lamps by their electrical 

 data." {Comptes Bendus, November, xcv, p. 946.) 



5. Electro -Chemistry. 



The weight of water decomposed by a current whose strength is one 

 electro-magnetic unit, passing for one second of time, has been variously 

 stated. "Weber gives it as 0,9376 milligram; Bnnsen, as 0.9205; Cas- 

 selmann, 0.9387 ; Joule, as 0.9222; and Cazin, as 0.9372. Mascart has 

 undertaken a new determination of this value. To this end he submit- 

 ted to electrolysis water acidu'ated with phosphoric acid and solutions 

 of silver nitrate and copper sulphate. For the measurement of current 



