420 Intelligence and Miscellaneous Articles, 



where the last term depends upon intermolecular forces, or what 

 Clausius called the Virial, and Laplace denoted by K. 



Besides this simple matter of separate gases, a much more 

 interesting thing about mixed gases was discovered by Clerk 

 Maxwell in 1860 (January, Phil. Mag.) — namely, that when a 

 number of independent gases are thoroughly mixed in the same 

 box, the average molecular kinetic energy of every variety of mole- 

 cule is the same. This he found by direct mechanical reasoning, 

 for he showed that at every encounter of unequal elastic spheres 

 any initial or outstanding difference of energy was diminished by 

 the collision; whence, since there are millions of collisions per 

 second in ordinary gases, a condition of average uniformity of 

 distribution of energy is soon attained. 



From this law of Maxwell's the law of Avogadro, or a slight 

 extension thereof, immediately follows ; viz., that in any given 

 volume of such a mixture, the number of molecules of each gas is 

 proportional to that gas's individual pressure. 



The proof is as follows: — p = ^nmu 2 , mu 2 = const., therefore n 

 varies with p ; and this holds for every gas separately, provided 

 that there are no intermolecular forces, chemical or other. 



I am, Gentlemen, 



Tour obedient servant, 



March 1, 1894. Oliver J. Lodge. 



AN EXPERIMENT TO PHOTOGRAFH THE SPECTRUM OF 

 LIGHTNING. BY G. MEYER. 

 In taking a photograph of the spectrum of lightning by means 

 of a spectroscope provided with a slit, the difficulty is met with 

 that, in order to obtain a satisfactory result, the lightning must be 

 exactly in the prolongation of the optical axis of the apparatus. 

 By the following arrangement this limitation may be removed. 

 In front of the object-glass of the photographic apparatus arranged 

 for parallel rays, a glass refraction-grating is fixed. An image 

 of the lightning is then obtained on the plate, produced by 

 the rays which traverse the grating without change of direction, 

 and, on each side of this, images which owe their origin to the rays 

 forming the lateral spectra. The number of images in each 

 lateral spectrum is equal to the number of bright lines occurring 

 in the spectrum of the lightning. During a storm at night in 

 May of this year (1893) I made an experiment of this kind, for 

 which a small hand camera was used ; in front of the object-glass 

 (an ordinary landscape-lens of 10 cm. focal distance) a glass grating 

 of 0*0256 mm. breadth of grating, was fixed. Two plates were 

 exposed. The first shows two flashes, and right and left of each 

 of them an image which corresponds to the lateral spectrum of 

 the first order. In the present case this only contains one line. 

 On the second plate were more flashes, and in the immediate 

 neighbourhood of the horizon, above which projected roofs of houses 

 with various chimneys. Between the chimneys a strong flash is to 

 be observed, with its two lateral spectra. I measured the angle 



