2. That they possessed, as a trade or profession, the arts of deception. Not only by the use of chemical preparations, but by other means, they pretended to work miracles in the dark ages of science. However degraded these persons may seem, they yield in vice to another class, who practised the art of poisoning, and who kept it so profound a secret, that few then understood the effect of the now common, vegetable, and mineral poisons. Who could have been more infamous than the celebrated female poisoner, Tophania? CHAPTER II. OF THE SUBSTANCES USED IN THE FORMATION OF FIRE-WORKS. _Sect. I. Of Nitrate of Potassa, or Saltpetre._ Nitrate of potassa, nitre, or saltpetre, is composed, as its name expresses, of nitric acid, and potassa. When pure, it contains, according to Kirwan, potassa 51.8, nitric acid 44, and water 4.2 in the hundred. This salt, when pure, or even mixed with other saline substances, is recognised by placing it on hot coals. Slight detonations, and a hissing noise, with a vivid combustion take place. It is also decomposed by sulphuric acid, and the nitrous vapour is apparent from its smell and colour. Nitrate of potassa crystallises in six-sided prisms, terminated by six-sided pyramids. Its specific gravity is 1.933. Its taste is sharp and cooling. One part is soluble in seven parts of water, at the temperature of 60 degrees, and in rather less than its own weight of boiling water. It melts in a strong heat, and by cooling congeals into an opaque mass, called _crystal mineral_, or _sal prunelle_. Exposed to a red heat, it disengages _oxygen gas_, and passes to the state of a nitrate; at a higher temperature, this is decomposed, and oxygen, azote, and a portion of nitrous acid, which has not been decomposed, are evolved. What remains is potassa. When projected on ignited coals, it burns brilliantly. Detonation also ensues by mixing nitre and charcoal, and throwing the mixture into a red-hot crucible. The residuum is carbonate of potassa. Fourcroy (_Système des Connoissances Chimiques_, Tome iii, p. 124.) observes, that metals, with nitrate of potassa, will decompose this salt, and produce different coloured flame, extremely brilliant, on which account such substances are used in fire-works.