It now remains only to mention two non-metallic elements that are nearly always present in crude iron, but which the metallurgist strives to eliminate, as they are in general very injurious to the quality of the material even when their amount is very small. The first is _sulphur_, well known as _brimstone_, also as _flowers of sulphur_, a yellow coloured solid, which burns in the air. The product of the combustion is an invisible gas of a readily recognized pungent odour: this is an acid-forming oxide containing equal weights of sulphur and oxygen. There is another oxide in which the weight of oxygen is one and a half times that of the sulphur, and this is the radicle of the very active _sulphuric acid_ or _oil of vitriol_. Sulphur, like oxygen, unites with most of the other elements, forming compounds called _sulphides_. Of these the iron compound called _pyrites_ is the best known, and its occurrence in coal prevents the use of that material as fuel in contact with iron or other metals. _Phosphorus_ is an element that occurs naturally only in combination; in its separated state it is a very inflammable solid. It combines directly with other substances and is taken up by some fused metals in large quantities. In many cases a very small proportion of it existing in a metal greatly modifies the properties of that metal. Phosphorus forms several oxides, and these are radicles of powerful acids, among which is _phosphoric acid_ that combines with basic oxides to form _phosphates_.