364 RECENT PROGRESS IN PHYSICS. 



the presence of oil does not have this effect; that is, when oil is in the 

 escape tube, electricity is developed if the water be slightly acid or 

 saline. 



I do not, however, see among these facts a single one opposed to the 

 view which seems to me to be at least more natural, namely: that 

 we have not to consider the friction of the oil on the sides of the 

 tube, but that of water on the sides affected by the oil ; a view which 

 Faraday does not entirely exclude, when he says : " It is very probable 

 that when wood, glass, or even metal is rubbed by these oily currents, 

 the oil may be considered as rubbing not merely against wood, &c., 

 but against water also," &c. 



When, from a vessel containing compressed air, a jet was caused to 

 impinge upon a cone of wood or brass, placed in Fig. 3. 



front of the opening, as shown in fig. 3, there loas ^ 



no indication of electricity as long as theairivas ^\t^^»p^rr-dI^M 



perfectly dry ; hut ivhenever the air ivas moist ^the ^ ' '-'-^^^' 

 cone became negative. Faraday ascribes this ex- 

 citation of electricity to the particles of water 

 which were condensed by the expansion and 

 cooling of the air striking against the cone. These particles were 

 visible both in the mist which appeared and by their moistening the 

 surface of the wood or metal. 



If the current of air carried particles of water which it had taken 

 up in its course against tlie cone, the latter, as might have been ex- 

 pected, became negatively excited. 



If the current of air carried with it the powders of different sub- 

 stances, these, too, were found to excite electricity. Flowers of sulphur, 

 lor instance, made wood and metal negative, pulverized quartz made 

 both positive. Other substances, such as pulverized resin and gum, 

 gave variable results. 



§ 8. Excitation of electricity by the escape of liquid carbonic 

 ACID. — On a strong glass support, by means of a wooden attachment. 

 Jolly insulated Natter er s condensation apparatus, with the exit pipe 

 directed downwards. When the opening was unscrewed, and the 

 liquid carbonic acid escaped, the apparatus became electric, and small 

 sparks could be drawn from it. 



§ 9. Armstrong's hydro -electric machine. — A description of this 

 machine will be found in my Lehrhuch der Physih, (Miiller's Physics.) 

 Compared with the largest and best plate machines, it does not excel 

 80 much by a greater tension, as by its affording a far greater quan- 

 tity of electricity. The length of the sparks is not greater than in 

 the most remarkable plate machines, but experiments which require, 

 in a short time, a great quantity of electricity, are rendered much 

 more striking by the hydro-electric machine. 



The greatest power is developed when the electricity is drawn ofT 

 in the form of a current without disruptive discharge. Thus the true 

 electrolytic decomposition of water, which had never before been ac- 

 complished unequivocally by frictional electricity, was performed in 

 the clearest and most distinct manner by the hydro-electric machine. 



Ten wine glasses were arranged in a row. They contained — 



