RECEXT PROGRESS IN PHYSICS. 359 



perature at 170 to 180° cent, gives out a considerable quantity of 

 water, and after cooling insulates well. 



§ 4. Electricity of rubbed glass. — It is well known that the kind 

 of electricity which glass receives by friction de])ends upon the rub- 

 bing substance. But Heiiitz, (Pogg. Ann., LIX, 805,) has further 

 shown that, by various means, glass may bo brought into such a con- 

 dition that by a slight rubbing it becomes negative, with substances 

 which, under ordinary circumstances, make it positive. 



If a glass rod be passed several times through the flame of a spirit 

 lamp, (whereby every trace of adhering electricity must be dissipated,) 

 and then rubbed gently with cloth, which ordinarily renders it posi- 

 tive, it becomes negative, and it is only after a continued and stronger 

 friction that positive electricity appears. 



It is not the heat of the glass rod which produces this effect, for if 

 after having been passed through the flame the rod is allowed to be- 

 come perfectly cold, or even laid aside for several days, it still becomes 

 negative by slight friction with cloth. 



This experiment shows that heat is not the immediate cause of the 

 above mentioned phenomenon, but it might be possible that the heat 

 of the flame was the cause of the condition of the surface of the glass, 

 by virtue of which it became negative by slight rubbing. But Heintz 

 has shown that even this is not the case. 



If a perfectly clean glass rod be wrapped in tin foil, or put into a 

 glass tube, and then held in the flame of a spirit lamp, so that the 

 flame does not touch it, but still heats it, the above mentioned pecu- 

 liarity does not appear, even if the temperature has been carried to a 

 high degree. 



In order to give to glass this peculiar property^ it is not necessary 

 to hold it within the flame, it is sufficient to pass it back and forth at 

 a distance of about three inches above the top of the flame of a good 

 spirit lamp with double current of air. 



To clean the glass rod properly it should be washed with a solution 

 of caustic potash, and rinsed with distilled water. 



Other flames produce the same effect as that of alcohol. 



The chemical action of the products of combustion cannot be the 

 cause of this phenomenon, for steam does not produce it, but the flame 

 of burning hydrogen does, and in this case nothing but the vapor of 

 water is produced. 



If a glass rod be dip])ed into concentrated sulphuric, muriatic or 

 nitric acid, and rinsed after its removal with distilled water, until the 

 drops no longer show an acid reaction, the adhering water thrown off", 

 and what still remains allowed to evaporate — the rod acts precisely in 

 the same way as it would have done if it had been passed through the 

 flame of a spirit lamp, it becomes negative by friction. 



Alkalies do not act like the acids, they cause the glass rod to become 

 decidedly positive. 



There is a great difference between the various specimens of glass 

 in regard to the facility with which they assume the above described 

 condition. 



Upon rock crystal, calcspar, gypsum and heavy spar, the flame has 

 the same action as upon glass. 



