154 Progress of Invention. 



minutes to light, and then placed in contact with sensitized paper 

 in the dark, will give rise, on development, to an image. Applying 

 this principle, we shall find that a latent image must result when 

 iodide of silver, for example, is exposed to light along with some 

 substance capable of absorbing light, and retaining it for a sufficient 

 length of time, and is then, before the light has been all emitted, 

 placed in contact with the requisite organic matter. Tbe condi- 

 tions necessary for the production of the chemical changes indis- 

 pensable to the obtaining a picture are fulfilled ; since the iodide of 

 silver and the organic matter are simultaneously in presence of 

 light, though not of the luminous body. 



Electric Condition of Different Parts of the Earth. — It 

 has been found that, if one extremity of a long wire is inserted in 

 the earth at one place, and the other in the earth at another place 

 which is higher than the former, a current of electricity will con- 

 stantly traverse the wire from the lower to the higher level. This 

 fact has been recently explained by M. Matteucci, and his explana- 

 tion has been experimentally illustrated by him. It is a conse- 

 quence of the positive electricity constantly carried towards the 

 poles by the wind, and the inductive action of this electricity on 

 the earth. The stratum of air near the earth, especially over seas 

 and lakes, is continually being neutralized, but the neutralization, 

 goes on more slowly and imperfectly in mountainous and even 

 slightly elevated regions. Hence the mountain and the air in con- 

 tact with it are in opposite electrical states of greater intensity, and 

 thus it is that clouds are so strongly attracted by mountains. The 

 mountain and the plains are negatively electrified, but not in an 

 equal degree ; and if their electricity is not in a static condition, a 

 conductor must lead a current from one to the other. They are not 

 in a statical condition ; for neutralization of the air in contact with 

 the earth is constantly taking place, and electrical equilibrium is 

 therefore constantly being disturbed. This is illustrated by M. 

 Matteucci by a very simple and satisfactory experiment. He places 

 a globe, made of porous earth or of wood, covered with moistened 

 blotting-paper, on an insulating stand ; fixes a small metallic disk 

 on the highest part of the globe, and in contact with the moistened 

 blotting-paper, and a similar disk from fifty to ninety degrees from 

 the first ; then, having connected the plates with a galvanometer, 

 he electrifies the globe. No current is, under these circumstances, 

 indicated by the galvanometer ; for the electricity of the globe is 

 in a static condition. He next suspends, by an insulator, a small, 

 slightly concave metallic plate over the higher disk, and about an 

 inch from it. This concave plate represents the atmosphere ; and 

 if it is placed in connection with one conductor of the electrifying 

 machine, and the globe with the other conductor, so as to oppositely 

 electrify the globe and the plate, the-galvanometer will, on turning 

 the electrifying machine, ;it once indicate the presence of a current. 

 The electric condition of the globe is different at different parts; 

 and it is no longer static, for the electricity of the globe is con- 

 stantly passing to and neutralizing that of the plate. It is not 

 always necessary that the extremities of a long insulated wire, in- 



