450 Dr. C. Sclmflmeutl's Remarks 



same as when it comes in contact with the outside, except 

 the inside of the sheet iron, by a process of oxidation, becomes 

 a non conductor of electricity in respect to the outside. An 

 incrustation, according to Mr. Armstrong's account, was found 

 in the boiler only as high as the water reached ; but in boilers 

 in which the water becomes very muddy, and which, there- 

 fore, are apt to prime, a sort of thin incrustation is often 

 spread over the whole interior of the boiler as well as the 

 safety-valve, and therefore the state of the interior of the 

 steam-chamber ought to be very closely examined. 



The number of sparks obtained by Mr. Armstrong from 

 the boiler at a distance of a quarter of an inch, amounted to 

 between 60 and 70 per minute. If we assume the quantity of 

 water necessary for a 28-horse power high -pressure engine 

 to be 2'4<7 cubic feet per minute, supplied by two boilers; one 

 boiler evaporated, therefore, 1*23 cubic feet per minute, and 

 the evaporation of 35'5 cubic inches of water with two ounces 

 of Newcastle coal would be necessary to produce one spark 

 of a quarter of an inch length per second, a quantity of elec- 

 tricity which seems to bear no proportion with the small 

 quantity of electricity produced during simple evaporation on 

 a small scale. 



But Mr. Armstrong's experiments seem distinctly to indi- 

 cate that the electricity of the steam depends chiefly on its 

 density, and the great quantity of free electricity may, there- 

 fore, perhaps, be made sensible by the rapid expansion of 

 high-pressure steam, and may perhaps have some relation to 

 the quantity of free caloric becoming latent during the ex- 

 pansion of high-pressure steam. I scarcely need here mention 

 the observation of Mr. Hare, that the operation of his defla- 

 grator was entirely suspended by the operation of the com- 

 mon galvanic trough apparatus ; besides, all conductors of 

 electricity during mutual friction develope caloric, whilst 

 non-conductors of electricity, on the contrary, during mutual 

 friction, develope, instead of caloric, electricity. 



It seems to me a great question whether the electricity of 

 the steam was not in close connexion with the deposit, or 

 the induration of the deposit upon the plates of the boiler. 



I have already shown in an article on steam-boiler explo- 

 sions, published in the Mechanics' Magazine, that those in- 

 crustations were composed of a series of distinct layers, some- 

 times very easily separable, and which proves that the indu- 

 ration or crystallization of these layers, notwithstanding the 

 continuous evaporation and feeding, must have been occasioned 

 at certain intervals, and that one layer must have already 



