“Again, according to Ritter, ultramarine contains a double silicate, not only associated with polysulphide, but also with thiosulphate of soda; and Schülzenberger, on the other hand, considers that it is a mixture of a double silicate with sulphite and monosulphide of sodium. “Endemann considers that the blue colour is due to a ‘colour nucleus,’ consisting of unchanging proportions of aluminium, sodium, oxygen and sulphur, in each variety of ultramarine the proportion being different, while the rest of the sodium and aluminium and the whole of the silica merely act as a vehicle necessary to the preparation and existence of the colour. He considers that this ‘colour nucleus,’ in the case of white ultramarine, which he calls the ‘mother-substance in the manufacture of blue ultramarine,’ has the formula AlNa_{4}O_{2}S_{2}. By the action on two molecules of this of sulphurous acid gas, Na_{2}O is removed, and green ultramarine Al_{2}Na_{6}O_{3}S_{4} is formed, which then, by the action of oxygen, which forms sodium sulphate, passes into the pure green compound, having the formula Al_{2}Na_{4}O_{3}S_{3}. In the ‘indirect process’ of manufacture, green ultramarine is converted into blue by being burned with sulphur. By this means Endemann considers that more sodium and sulphur are removed, and blue ultramarine Al_{2}Na_{2}O_{3}S_{3} is formed. He considers that the other portion, not included in the ‘colour nucleus,’ differs in different samples. In one which he mentions it has about the composition 3Al_{2}O_{3}.5Na_{2}O.16SiO_{2}.