Polytechnic Association. G95 



steam is indicated by the name given to this apparatus, which is 

 called the hydro-electric machine. 



These considerations establish the probability of the evolution and 

 accumulation of the gases, oxygen and hydrogen, in the upper part 

 of the boiler, while the process of generating steam is going on, 

 together with the production of electricity in large abundance. 



2. In the state of mechanical mixture those gases are innocuous, at 

 most operating only by increasing the amount of pressure; but when 

 brought in contact with flame, red heat or the electric spark, no sub- 

 stances are more violently explosive. In the generation of steam, 

 there is probably a double evolution of electricity ; first in the conver- 

 sion of water into steam, and again in its decomposition into its con- 

 stituent gases. The immense quantity eliminated when it reaches 

 the upper part of the boiler spreads, by a law regulating its diffusion, 

 over the surface and is gradually conducted off; but the always 

 present danger is of a surcharge. For bodies will only contain a 

 certain quantity of electricity, there being, as has been demonstrated, 

 a limit to their capacity in this respect. 



A metallic ball charged from different sources with electricity, on 

 the attempt to surcharge it, will discharge itself through the air into 

 the nearest conducting body ; a spark, describing apparently a zigzag 

 course, will be observed traveling with immense velocity and an 

 audible sound, and capable of giving a severe and dangerous shock. 

 (9 Penny Cyclopedia, 337.) It is these surcharges, as we have sup- 

 posed, which, in that great natural laboratory the thunder cloud, 

 cause the lightning, and they occur, we do not doubt, occasionally, 

 and are always in certain electrical conditions of the atmosphere 

 liable to occur, in the operation of the steam boiler. The electricity, 

 gathering and spreading over the inner surface of the boiler, passes 

 away by means of the metallic connections into the exterior air more 

 or less rapidly, and, when the safety valve is much used, very rapidly 

 with the escaping steam. But the interior surface of the boiler,, 

 above the water line, is kept by the heated steam excessively dry, 

 and its conducting power thereby modified ; and when the exterior 

 air is very dry, the diffusion of electricity is much obstructed. We 

 can readily comprehend how the electricity may, under these circum- 

 stances, collect in such quantity on the surface of the boiler as to over- 

 charge it, when the electric spark is sure to break forth (passing into 

 the water beneath, which is a good and the nearest conductor), and 

 fire and explode the accumulated gases. In confirmation, it is 

 recollected that the most disastrous explosions of steam boilers have 



