All the bodies in the preceding Table, excepting Nos 18 and 19, were prepared in the following manner : They were melted in the proper quantities in iron ladles. mul as soon as they were taken off the fire, they were set by to cool and harden. The ladle was then placed on the fire for a short time, till the substance was melted at its bottom and sides, and, by inverting the ladle, the sub stance was easily taken out, and had the form of nearly the section of a sphere, both the convex and the plane surfaces being polished in cooling.
When any of these substances were taken out of the ladle, and their convex surfaces hardened, they did not exhibit any electrical indications, till their temperature was diminished to nearly that of a hen's egg when new ly laid. The electricity gradually increased, and when they were cold, it was nearly 10 times greater than at the preceding temperature. In order to preserve these bo dies in a state of attraction, he wrapped up the larger ones in white flannel, or black worsted stockings ; and the smaller ones in white paper, and placed them all in a large fir box.
The cylinder of sulphur, No. 18, was formed by melt ing the sulphur, and pouring it into a cylindrical glass vessel, which had been previously heated to prevent it from cracking. As soon as the sulphur was hardened, it fell out of the glass by inverting it, from having con tracted during cooling, and had a surface as perfectly polished as the glass itself. The large cone of sulphur was made in a similar manner, by melting the sulphur in a large drinking glass. Mr Gray examined all the bodies 30 days after they were made, and found that they attracted as vigorously as they did at first, and some of them did not lose their attraction till after 4 months. The cone of sulphur, No. 19, began to attract about two hours after it was taken out of the glass ; and the glass itself attracted likewise, but very feebly. On the fol lowing day, when the sulphur was taken out of the glass, its attraction was very strong, and that of the glass imperceptible. The cone of sulphur was in these cases placed with its base on the top of the fir box, where the other electric bodies lay, and the glass placed over it ; but finding this place inconvenient, Mr Gray removed it to the table, between the two windows of his chamber, and whenever the glass was taken off, it at tracted at as great a distance as the sulphur. After wards the glass attracted at a less distance than the sul phur. The cake of sulphur, No. 20, was laid with its
flat side downwards upon a table, and though it had no covering of any kind upon it, the attraction of this, as well as that of the other substances, always varied with the weather ; but the attraction of the cake of sulphur was never more than one-tenth of that of the cone. Mr Gray observed all these attractions, by the action of the electrical bodies upon a fine white thread tied to the.end of a stick.* Experiments similar to those of Mr Gray were made by M. Wilke of Rostock, who called the electricity which was in this way produced, spontaneous electricity. }Epinus made experiments ou the same subject ; but as we have already given a sufficiently full account of these in our History of Electricity, p. 246, 247, it is needless to repeat them in this place.
Mr Henley repeated the experiments of Mr Gray, and obtained nearly similar results. He discovered that electricity was exhibited by chocolate, when it was cool ed in the tin pans into which it is received. At first the electricity is strong, and it is retained for some time after it is taken out of the pans, though it soon loses it by handling. When it is again melted and allowed to cool, the electrical virtue is restored, but not to its former strength. After the third or fourth melting, however, the electricity is extremely weak. When the chocolate is mixed with a little olive oil before it is poured out of the pan, it then becomes strongly electrical.
These experiments of Mr Henley were repeated and verified, in 1784, by M. Pabst, and in 1787 by 111. Lip hardt of Konigsberg. The last of these writers being at one time occupied in preparing chocolate, placed some cakes upon one another, and having held a bundle of silk threads within two inches of them, they w ere at tracted with great velocity, and adhered to the cakes. M. Liphardt also found that talc melted and cooled gave electrical symptoms. Thinking that electricity could not be produced without friction of some kind, M. Lip hardt took four ounces of warm and liquid chocolate, and having placed them upon an iron plate, he brought near the silk threads, but observed no indications of elec tricity. He then put the mass into shape, and having struck it well against a flat surface, as is the custom in extending the chocolate, he took it warm out of the shape, and found it to be electrical.