Prof. Matteucci on a new Experirnent in Electricity. 321 



the exposition of your theory. This lecture, reported by one 

 of the eleves of this school, will appear in our journal, the 

 Cimento. 



After having repeated your beautiful experiment of the 

 pieces of silk suspended in essential oil of turpentine, and 

 which unite together between the two electrified balls, I spoke 

 of the figures obtained by sprinkling a mixture of two kinds 

 of powder, sulphur and minium, upon a plate of resin, over 

 which the rod of a charged Leyden phial had previously been 

 passed. The other experiment, which appears to me to be 

 sufficiently decisive, is this. I separate a piece of mica into 

 very thin laminae, not thicker than the finest paper; I cut these 

 into small squares with a pair of scissors, leaving a small ap- 

 pendage on one side. I pile these little squares one upon the 

 other, subjecting them to a strong pressure, taking care how- 

 ever that the appendages do not correspond. This done, I 

 begin by showing, that on separating these laminae one from 

 the other, and rapidly approaching them — first with one sur- 

 face, and then with the other — to the nob of an electroscope 

 sufficiently delicate for the purpose, and furnished with a dry 

 pile, either there was no sign of electricity, or if in any case 

 there were signs, they were undetermined and varied, and in 

 all probability owing to friction. After this I replace the 

 little squares of mica one upon the other, as described above, 

 and lay the heap upon a metallic plate, in communication 

 with the ground ; and on the upper lamina of mica I place a 

 little disc of tin. 1 next put this tin disc in communication 

 with an electrical machine, and after a few turns I disarrange 

 the pile; taking the little squares of mica, one by one by their 

 appendages, and applying them to the electroscope. I pre- 

 sent first one surface and then the other; and between each 

 experiment I put the nob of the electroscope in communica- 

 tion with the ground, so as to discharge it. It never fails to 

 happen that the two surfaces of each of the laminae are in op- 

 posite states of electrical excitement, that is, that surface of 

 every lamina is positive which is turned towards the positive 

 coating, the other surface of every lamina which is turned to- 

 wards the negative coating is negatively excited. 



I have only repeated this experiment a very few times, and 

 cannot precisely tell you the laws of this pojarization. It has 

 appeared to me that a certain determined thickness of the 

 lamina of mica is necessary to the production of the phaeno- 

 menon, and therefore that a too thick or a too thin lamina 

 would be unfit. I have also imagined that the charges de- 

 creased from the extreme lamina towards the middle layers, 

 and so it appears to me it should be. Engaged in the conti- 



Phil. Mag. S. 3. Vol. 26. No. 173. April 184.5. Z 



