It has not been determined yet what are the conditions and circumstances of the change which takes place by the ac tion of the air at the face of the rubber, nor why the surface of the glass should become positive when rubbed with one kind of rubber, as for example the human hand; and negative, if rubbed with ano ther kind, such as cat-skin, or flannel ; nor why glass, deprived of its polish, be comes negative with rubbers, which would have rendered smooth glass posi tive. The most rational conjecture seems to be, that the surface which is most heated in consequence of its roughness, or the relative smallness of its dimen sions, acquires the negative state.
There is a certain velocity of rotation, which is about five feet per second, at which the excitation of electricity by a cylinder nearly vanishes; hut it returns again the moment the velocity is dimi nished. Some, who maintain the exis tence of a material cause of heat, or calorie, are disposed to consider electri city as one of the states of caloric, in which the matter of heat can pass through bodies without raising their tempera ture, and with much greater velocity than that by which temperature is com municated.
From the imperfect knowledge we pos. sess respecting excitation, it is very diffi cult for the most experienced electrici ans to excite a cylinder with certainty and power. If the cylinder be greased all over with tallow, and then turned for some time in contact with the cushion, the silk flap being thrown back, and an amalgamed leather be applied and rubbed about upon the surface of the cylinder in motion, electric sparks are soon produced in abundance ; and if the silk be then thrown again into contact with the cylin der, the excitation will, in general, be strong ; but it is seldom so strong at the first time of exciting, as it proves to be after the expiration of a day or more. It seems as if' the amalgam and tallow re quired a considerable time of working to be brought into the best state for ex citation.
In order to judge of the degree of in tensity of an excited cylinder, we must have recourse to some standard of the quantity of effect produced by the fric tion of a given surface. It has not been shewn that much, if any thing, depends on the thickness of the glass, though some kinds of glass are more excitable than others, and some not at all so. If a coated electric jar be taken of about one twentieth of an inch in thickness, (see Jron, electric,) a cylinder or plate mode rately excited, will require fifty or sixty square feet to pass the cushion, in order to charge one foot of the coated glass, so as to explode over a rim of three inches, which is as much as can be admitted with out danger of the explosion breaking through the jar. If tl.e excitation be stronger, the charge may be made by the friction of thirty feet to one of the jar ; and the strongest excitation the edi tor has ever known has been by the fric tion of fourteen square feet of a cylinder to charge one foot of glass. But as the labour increases by adhesion of the cushion, the stronger the excitation, it seems as if the strength of a man would be more profitably employed in turning two or more plates, or cylinders, at the intensity of thirty feet, than at any higher intensity : besides which, this power is less variable, and may last five or six hours without requiring fresh amal gam.
The vulgar notion of electricity is, that it is fire which passes in a spark from one body to another. From its passage through dense conductors, as well as through the air, it seems to move with extreme ve locity : and this may be sufficient, with out supposing it to be essentially lumi nous, to account for the ignited appear ance it affords, in all non-conductors, whe ther air, or oil, or glass, or wood, he. and even in metal, when the conductor is small. If oxygen be present, these bo dies will have their combustible parts burned ; and if not, a decomposition of those parts which are ignited may ensue.