The next in order is what is called 60 per cent. white. It {178} consists of a hard white mass, requiring considerable force to break it. Though whiter in appearance than cream caustic, it is in reality less pure, as it contains a considerable quantity of salt. It is made by continuing the evaporation of the caustic liquor to a further point than is the case with cream caustic, the result being that the whole of the water is driven off. A small quantity of nitre is then added to oxidise the sulphides and other compounds which impart the colour to cream caustic. Common salt is then added to reduce its strength to 60 per cent., in order to satisfy the whim of the consumer, who insists upon an article of a given definite strength. On this account it is somewhat inferior to cream caustic, and is, moreover, more expensive. The highest quality of all is white 70 per cent. This resembles white 60 per cent. in appearance, but is much purer, and more expensive. A very pure form of caustic soda has within the last few years been introduced by the Greenbank Alkali Company. It consists of small fragments, more or less crystalline, of almost pure sodium hydrate. It can be packed in wooden casks, and it dissolves very readily in water. It contains 76 per cent. of alkali. It is, however, much too costly for boiling purposes; it might, however, be used for the preparation of rosin size. The following analyses of different forms of caustic soda and their relative prices will possibly be of interest:— -----------------+-----------------+-----------------+--------------- | (Davis.) | (Davis.) | (Morrison.) | White (70 Per | White (60 Per | Cream (60 Per | Cent. Na_{2}O). | Cent. Na_{2}O). | Cent. Na_{2}O). -----------------+-----------------+-----------------+--------------- Sodium hydrate | 89·60 | 75·25 | 70·00 Sodium carbonate | 2·48 | 2·53 | 5·00 Sodium chloride | 3·92 | 17·40 | 7·00 Sodium sulphate | 3·42 | 4·40 | 2·00 Sodium sulphide | 0·02 | 0·03 | — Sodium silicate | 0·30 | 0·30 | — Sodium aluminate | trace | trace | — Water | none | none | 15·80 Insoluble matter | none | none | 0·20 -----------------+-----------------+-----------------+--------------- {179} Their relative prices per ton f.o.b Liverpool, in December 1886, were:—70 per cent. white, 8_l._ 5_s._; 60 per cent. white, 7_l._ 5_s._; 60 per cent. cream, 7_l._