ELECTRICITY. 



497 



Descriptive 



Bennet's 

 method of 

 forming the 

 configura- 

 tions. 



Formation 

 of the resi- 

 nous plate. 



Preparation 

 of the pa. 

 per. 



Way of 

 lojuiig 

 tlit powder. 





in about an ounce of gum arabic, and a lump of alum 

 about as big as a large nut ; let it boil about two hours, 

 or till the water is strongly coloured ; strain off the ex- 

 tract into a broad dish, and set it in an iron oven, where 

 it is to remain till all the water be evaporated, which 

 with me was effected in about twelve hours ; but this 

 depends on the heat of the oven, which should not be 

 so hot as to endanger its burning. Sometimes I have 

 boiled the strained extract till it was considerably in- 

 spissated before it was placed in the oven, that it might 

 be sooner dry. 



When it is quite dry but not burnt, scrape it out of 

 the dish, and grind it in a mortar till it be finely pul- 

 verized. In doing this, it is proper to cover the mor- 

 tar with a cloth, having a hole through to prevent the 

 powder from flying away and offending the nose, and 

 also to do it out of doors if the weather be dry and calm, 

 that the air may carry away the powder necessarily 

 escaping, and which otherwise is very disagreeable. 

 When ground fine, let it be sifted through a fine 

 hair sieve, returning the coarser part into the mor- 

 tar to be ground again. When the grinding and sift- 

 ing are finished, the powder is ready for use. The re- 

 sinous plate I have mostly used, was composed of five 

 pounds of rosin, half a pound of bees-wax, and two 

 ounces of lamp-black, melted together, and poured upon 

 a board sixteen inches square, with ribs upon the edges 

 at least half an inch high, to confine the composition 

 whilst fluid : thus the resinous plate was half an inch 

 thick, which is better than a thinner plate, the figures 

 being more distinct. After the composition is cold, it 

 will be found covered with small blisters, which may 

 be taken out by holding the plate before the fire till 

 the surface be melted, then let it cool again, and upon 

 holding it a second time to the fire, more blisters will 

 appear ; but by thus repeatedly heating and cooling the 

 surface, it will at last become perfectly smooth. Some 

 platen were made smaller, and the resinous composition 

 /.-onfineil to the form of an ellipsis, a circle, or escutcheon, 

 by a rim of tin half an inch broad, and fixed upon 

 a board. 



The next thing to be done is to prepare the paper, 

 which is to be softened in water, either by laying the 

 pieces upon each other in a vessel of cold water, or first 

 pouring a little hot water upon the bottom of a large 

 dish, then laying upon it a piece of paper, so that one 

 edge of the paper may lie over the edge of the dish, to 

 remain dry, that it may afterwards be more conveni- 

 ently taken up. Then pour more hot water upon its 

 upper surface. Upon this place another piece in the 

 same manner, again pouring on more water, and thus 

 proceed till all the pieces are laid in. By using hot 

 water, the paper will be more softened in a few mi- 

 nutes than if it remains in cold water a whole day. 



When the figures are to be made, the resinvus plate 

 must lie horizontally, whilst the electricity is communi- 

 cated, if the experiment requires any thing to be placed 

 upon the plate : but it is convenient afterwards to hang it 

 up in a vertical position whilst the powder is projected, 

 lest too much powder should fall where it is not required. 



A little of the powder may be taken between a finger 

 and thumb, and projected by drawing it over a brush; 

 or, what is better, a quantity of powder may be put 

 into the bellows and blown towards the plate. When 

 the figure is sufficiently covered with powder, let the 

 plate be again laid hori/ontally upon a table ; then take 

 one of the softened papers out of the water by its dry 

 edge, and lay it carefully between the leaves of a book, 

 pressing the book together, and let it lie in this situa- 

 tion about half a minute. Then remove the paper to a 



VOL. VIII. PART II. 



dry place in the book, and press it again about the Descriptive 

 same time, which will generally be sufficient to take Electricity.^ 

 off the superfluous moisture. Then take up the paper S "~V"'' 

 by the two corners of its dry edge, and place the wet Method of 

 edge a little beyond the figure on the resinous plate, 

 lowering the rest of the piece gradually till it covers 

 the figure without sliding ; then lay over it a piece of 

 clean dry paper, and press it gently ; let it remain a 

 short time, and then rub it closer to the plate with a 

 cloth, or, which is better, press it down by means of a 

 wooden roller covered with cloth, taking care that the 

 paper be not moved from its first position. When the 

 paper is sufficiently pressed, let it be taken up by its 

 dry edge, and laid upon the surface of a vessel of water 

 with the printed side downwards ; by this means the 

 superfluous powder will sink in the water, and the 

 figure will not be so liable afterwards to spread in the 

 paper. After the paper has remained on the water 

 during a few minutes, take it up and place it between 

 the leaves of a book, removing it frequently to a dry 

 place. If it be desired tliat the paper should be speedi- 

 ly dry, let the book-leaves in winch it is to be placed 

 be previously warmed, and by removing it to several 

 places it will be dry much sooner than by holding it 

 near a fire, and without drawing the paper crooked. 

 By the above process, it is obvious, that leather, cali- 

 co, or linen, as well as paper, may be printed with 

 these figures, and the effects of the diffusion of electri- 

 city upon a resinous plate be exhibited to those who 

 have not leisure or inclination to perform the experi- 

 ments." The experiments of Lichtenberg have been 

 varied and extended by Van Troostwyk and Cavallo, 

 but we can find room only for a reference to their 

 works. See Lichtenberg, Nov. Comm. Gutting. 1777, 

 vol. viii. p. 168. Symmer, Phil. Trans. 17. vol. li. p. 

 371. Beccaria Lettere del? EUettricismo, p. 74- Cavallo's 

 Phil. Trans. 1780. p. 15. Gough, Nicholson's Journal, 

 vol. xxxiii. j). 170'. Ezekial Walker, Phil. Magazine, 

 vol. xlii. p. 161. Singer's Elements of Electricity, p. 16". 

 Priestley's History of Electricity, p. 675. Ingennousz, 

 Phil. Trans. 1778, vol. Ixviii. p. 1022. Nairne, Phil. 

 Trans. 1780, vol. Ixx. p. 334. 



SECT. II. On the Chemical effects of Electricity. 



THE effects of electricity as a powerful chemical agent, 

 constitute one of the most interesting portions of the sci- 

 ence. As these effects, however, are produced to a 

 greater extent by the Galvanic apparatus, we shall have 

 occasion to treat the subject at greater length under 

 the article GALVANISM. At present we shall confine 

 ourselves to a few details respecting the action of the 

 electric fluid in inflaming combustible bodies, in fusing 

 and oxidating metals, in promoting the combination and 

 decomposition of different bodies, and in rendering them 

 phosphorescent. 



Exp. 1 . Having placed warmed alcohol or ether in 

 an insulated metallic cup, electrify tfie cup, draw a 

 spark from the bottom of the cup erther by the knuckle 

 or any other conducting body, and the alcohol will be 

 set on fire by the transmission of the spark. 



Eip. 2. Place powdered rosin, or phosphorus, or 

 camphor, on some cotton wool, and wrap it round one 

 of the knobs of a discharging rod. Then liaving cliar- 

 ged a Leyden jar, apply the naked knob to the ex- 

 ternal coating, and the covered knob to the ball of the 

 phial, and the rosin, phosphorus, or camphor, will be 

 set on fire by the discharge. Powdered rosin laid on 

 the surface of water may also be inflamed by passing a 

 charge over the surface of the water between two points. 

 9* 



On the che- 

 mical ef- 

 fects of 

 electricity. 



On the ef- 

 fects of clec- 

 tricily in in- 

 flaming bo- 

 dies. 



