C. GENERAL PHYSICS. 123 



length of two and a half feet, and the resulting electric cur- 

 rent deposits eight hundred grammes of silver per hour, re- 

 quiring a motive force of one horse-power. The experiments 

 made at London by this machine give it the first rank among 

 apparatus for producing electric light. A curious experiment 

 was performed with one of these machines at the Exposition 

 of Vienna. The principal magneto-electric machine being 

 driven by an air-engine on the Lenoir system, the electricity 

 thus produced was carried to the electro magnets of two 

 Gramme machines of more feeble dimensions, which, acting 

 as an electric motor, kept a small centrifugal pump in ac- 

 tion. Thus the mechanical effect of the electricity was by 

 the connecting wires carried to a great distance from the 

 motor. The principal advantage of the transmission of force 

 by electricity is found in the possibility of overcoming ver- 

 tical spaces which are inaccessible to the cables or belts 

 in use in manufacturing works. Bulletin Hebdomadaire y 

 XVI., 8. 



CRYSTALLIZATION ILLUSTRATED BY THE MICROSCOPIC 



PHOTOGRAPH. 



It is often very important in chemistry or in crystallog- 

 raphy to be able to seize exactly the delicate forms of crys- 

 tals, as examined by the microscope ; but the most conscien- 

 tious draughtsman can not always reproduce the fineness of 

 these crystallizations. According to Girard, many crystals 

 can be photographed. As a preliminary, it is necessary gen- 

 erally to dilute the solution to different degrees, and to al- 

 low one specimen of each kind of crystal to be formed by it- 

 self. AVe thus acquire the faculty of choosing that degree 

 of concentration where the crystallographic characters are 

 most appropriate to the methods of photography, either by 

 reason of the delicate grouping of the forms, or by their 

 transparency to the penetration of light. The most simple 

 method of photographing the crystals consists in the use of 

 an ordinary microscope, the body of which is placed in a 

 horizontal position. In a dark room upon a table near a 

 window the microscope is placed, and the object is illumi- 

 nated by a beam of light coming through the window from 

 a heliostat without. The luminous rays pass through the 

 thin film of water containing the crystallized salt, traverse 



