Steam developes Positive Electricity. 97 



become warm after repeated trials ; the only requisite be- 

 ing that peculiar condition of the steam which produces the 

 above-mentioned rushing noise. Now this noise appears to 

 be caused solely by the sudden boiling of the water, and the 

 conversion of a portion of it into a fine spray, as the steam 

 issuing under those circumstances and splashing against the 

 interior of the bell deposits a large quantity of water run- 

 ning down in drops, and often in streams, from the edge of the 

 bell. 



The boiler was insulated and the stop-cock was opened 

 with a dry folded silk handkerchief, but the omission of all 

 these precautions did not in the slightest degree affect the 

 results ; a proof that the electricity of the steam developed 

 in the glass bell could not be contained in the steam during 

 its passage through the 3-inch long metallic jet of the boiler, 

 as all electricity would have been deposited in this narrow 

 metallic passage. But the condensation of steam even into 

 the form of mist, seems not to be sufficient to produce elec- 

 tricity ; the condensation into liquid water seems to be indis- 

 pensable, at least in the present experiment, where the elec- 

 tricity appears to be due to the deposit of liquid water in the 

 bell solely, or perhaps in relation to the separation of this 

 liquid water from the steam. 



The circumstance of steam in a state of mist not being 

 capable of exhibiting traces of free electricity, seems to afford 

 us some clue to the elucidation of a hitherto mysterious phae- 

 nomenon, viz. that only certain clouds are capable of pro- 

 ducing thunder storms. A common cloud consisting only of 

 moisture seems to be analogous to a pure jet of steam in the 

 glass bell, both consisting of minute hollow water globules or 

 bubbles, leaving only a small deposit of moisture in the glass 

 bell, or in the air, which finally collects into small drops of 

 rain. But when the steam deposits rapidly a great quantity 

 of liquid water which in a thunder-cloud produces those well- 

 known heavy showers, electricity is set free in great quan- 

 tities, so that a jet of issuing steam from Marcet's boiler in 

 three seconds produced the same effects upon the gold leaves 

 of the electroscope as a feeble spark from an electric machine 

 with a glass plate of 9 inches, produced in damp weather. 

 1 have here only (o observe, that the sudden separation of 

 water in drops in thunder-clouds seems to be caused, as I ex- 

 pressed on a former occasion, by a sort of sudden compression 

 and cooling caused by the vehement currents of air towards 

 the centre of the thunder-cloud; because I found when I was 

 immersed in a thunder-cloud, that the hygroscopc during 

 Phil. Mag. a. 3, Vol. 18. No. 115. I'cb. isil. H 



