There are many other analyses in addition to those we have given and agreeing with them, so that there is no doubt as to the composition of ultramarine, but as to the manner in which the elements are grouped nothing is definitely known. Some chemists are of the opinion that the colouring principle of ultramarine is a sulphur compound of iron, whilst others oppose this view and consider that the colour is due to the combination of a double silicate of alumina and soda with an unknown sulphide of sodium. Although no blue or green compound is known of corresponding composition, the majority of chemists incline to the latter view. Experience has shown that the presence of iron in any of the materials used in the manufacture of artificial ultramarine is very dangerous to the success of the operation, and at the least considerably injures the beauty of the product. Although the manufacture of ultramarine is now very well known, it cannot be denied that some works produce a pigment of a shade which cannot be obtained by others. These works keep their method very secret, so that it is not possible to say with certainty whether they have introduced a process varying from that commonly known, or whether, by carefully watching the process, they have achieved great technical dexterity in the manufacture of this product. The latter appears the most probable, for in order to obtain a good result, many experiments, and an accurate knowledge of the raw materials, are necessary. The raw materials used are as follows: pure aluminium silicate, sodium sulphate, soda, sulphur, coal. The aluminium silicate is used in the form of fine china clay or kaolin, sodium sulphate and soda must be used in the anhydrous form, the sulphur is the ordinary commercial substance purified by distillation. Charcoal or coal containing little ash can be used. Whilst the remaining raw materials are always of similar composition, the china clay from different localities possesses a very varying composition. This substance must be carefully chosen. There is hardly any kaolin which is naturally of sufficient purity to be used without purification; it is well known that the china clay used for porcelain is subjected to a thorough preparation before it is used. Kaolin, like all clays, has been produced by the decomposition of feldspar; when the aluminium silicate so formed was able to deposit without foreign admixtures, that mineral was formed which is the purest of all clays and is called kaolin. The more foreign substances are mixed with the aluminium silicate the further is the clay removed from kaolin. The impurities which generally accompany the aluminium silicate are quartz sand, chalk and ferric oxide. We distinguish accordingly between kaolin, white clay or pipe clay, clay, and lastly marl, a clay containing much chalk.