210 M. Becquerel on the Development 



peculiar manner. It has long been known, that the more the 

 temperature of a body is raised, the greater is its tendency to 

 acquire resinous electricity by friction with a non-conducting 

 body. Thus, if the temperature of Iceland spar be sufficiently 

 raised, it may be made to acquire resinous electricity by a 

 slight pressure with the disk of cork. The following experi- 

 ment will also show the influence of caloric in electrical ex- 

 jieriments by pressure : — Take a very dry cork, and cut it in 

 lialf with a' very sharp instrument, and press the two parts 

 together by their newly Cut surfaces, — each of them will 

 usually acquire an excess of contrary electricity on being 

 withdrawn from compression: it will however be found, some- 

 times, that they have acquired no excess of electricity, however 

 great may have been the rapidity of their separation. In this 

 case, if the temperature of one of the two disks be raised by 

 warming it slightly at the flame of a taper, both will be imme- 

 diately electrified by the pressure. Two pieces of Iceland 

 spar of equal temperature are not more electric by pressure ; 

 a slight difference of temperature between them suffices to give 

 them the property of becoming electric. May we not conclude 

 from these two experiments, that in two bodies of the same 

 nature, of equal temperature, and in which the state of the par- 

 ticles of the surface is similar, — in two bodies, in short, which 

 are identically the same, no electricity can be developed by 

 pressure. It appears that this must be the case; for if every 

 thing be perfectly alike on each side, there is no reason why 

 one of the surfaces should take the vitreous rather than the re- 

 sinous electricity, or vice tvrsd : pressure, therefore, cannot 

 change the state of equilibrium of the two fluids which com- 

 pose natural electricity. If two disks of cork, taken from the 

 same piece, sometimes give out electricity upon pressure, it is 

 probably because the two surfaces are not identically the same: 

 in fact, unless the instrument with which they are cut, sepa- 

 rates them with exti'eme precision, it must follow that the state 

 of the molecules of the surfaces is not the san^e in both of 

 them. M. Dessaignes had already observed that a glass rod 

 is not excitable when plunged into mercury of the same tem- 

 perature; — it is the same with sulphur, with amber, and with 

 sealing-wax. There are, however, exceptions; for the same 

 philosopher discovered that paper, cotton, wax, and wool, are 

 always electric by contact, whatever precaution may be taken 

 to equalize their temperature. 



If we keep the temperature of one of the disks higher than 

 that of the other, the pressure, as we have just observed, in- 

 duces upon each piece of cork a different electric state; but if 

 the pressure lasts long enough for equilibrium of temperature 



U) 



