To finish this history it will be now proper to lay before the reader a kind of map of the present state of chemistry, that he may be able to judge how much of the science has been already explored, and how much still remains untrodden ground. Leaving out of view light, heat, and electricity, respecting the nature of which only conjectures can be formed, we are at present acquainted with fifty-three simple bodies, which naturally divide themselves into three classes; namely, _supporters_, _acidifiable bases_, and _alkalifiable bases_. The supporters are oxygen, chlorine, bromine, iodine, and fluorine. They are all in a state of negative electricity: for when compounds containing them are decomposed by the voltaic battery they all attach themselves to the positive pole. They have the property of uniting with every individual belonging to the other two classes. When they combine with the acidifiable bases in certain proportions they constitute _acids_; when with the alkalifiable bases, _alkalies_. In certain proportions they constitute _neutral_ bodies, which possess neither the properties of acids nor alkalies. The acidifiable bases are seventeen in number; namely, hydrogen, azote, carbon, boron, silicon, sulphur, selenium, tellurium, phosphorus, arsenic, antimony, chromium, uranium, molybdenum, tungsten, titanium, columbium. These bodies do not form acids with every supporter, or in every proportion; but they constitute the bases of all the known acids, which form a numerous set of bodies, many of which are still very imperfectly investigated. And indeed there are a good many of them that may be considered as unknown. These acidifiable bases are all electro-positive; but they differ, in this respect, considerably from each other; hydrogen and carbon being two of the most powerful, while titanium and columbium have the least energy. Sulphur and selenium, and probably some other bodies belonging to this class are occasional electro-negative bodies, as well as the supporters. Hence, when united to other acidifiable bases, they produce a new class of acids, analogous to those formed by the supporters. These have got the name of sulphur acids, selenium acids, &c. Sulphur forms acids with arsenic, antimony, molybdenum, and tungsten, and doubtless with several other bases. To distinguish such acids from alkaline bases, I have of late made an alteration in the termination of the old word _sulphuret_, employed to denote the combination of sulphur with a base. Thus _sulphide_ of arsenic means an acid formed by the union of sulphur and arsenic; _sulphuret_ of copper means an alkaline body formed by the union of sulphur and copper. The term _sulphide_ implies an _acid_, the term _sulphuret_ a _base_. This mode of naming has become necessary, as without it many of these new salts could not be described in an intelligible manner. The same mode will apply to the acid and alkaline compounds of selenium. Thus a _selenide_ is an acid compound, and a