The most common substance in this class is soda ash, or the more pure soda crystals. These two substances are regularly sold at very large profits to manufacturers, and receive very many names in the trade. One sample, commonly and extensively in use in and about London, consists simply of soda crystals roughly coated with litmus powder, and yet this simple addition gives a profit to the maker enormously greater than could possibly be obtained for the body if sold for what it in reality is. The action of this substance depends on the interchange of acids with the calcium sulphate, yielding a proportion of sulphate of soda, which principally remains in solution and passes off by the blowcock, but is also found in varying proportions in the incrustation.

The scale formed where soda crystals are used is frequently very hard, and difficult to remove. The chemical composition of this class of incrustation is given in Table I.

Acids and mixtures intended to liberate such, as oil. of vitriol and common salt (G. E. Davis, in papers read before the Manchester - Scientific and Mechanical Soc, 1879, p. 36), should never be employed, on account of the solvent action they have on iron, brass, etc. Barium chloride forms the insoluble sulphate of barium, which, sinking to the bottom of the boiler, adds greatly to the scale. Davis, in the paper above quoted, recommends " tripsa " (the tri-basic sodic phosphate), and states that he has obtained good results from its use. He, however, does not state the cost, nor does he make any mention of the commercial value of the by-products, which might be useful in manure manufacture, if obtainable in quantity.

Sulphites are occasionally employed. Morgan's compound belongs to this class. As far as Macadam can J earn, they act well with salt water, giving a soft sludge, which should be readily removed by the blowcock. He analysed the sediment from the boiler of a steam-tug which had been using Morgan's solution for about 10 days. The results gave an indication of the action of the compound. The boiler had been in

work for a week without any anti-incrustator, and a certain amount of hard scale had been formed, an analysis of which is also given: -

Table I. - Boiler Incrustations. - Carbonate Of Soda Used In Boiler

Table J. - (A) Boiler Incrustation. - Sea Water

(B) Sediment From Boiler. Sulphite Anti-Incrustator. - Sea Water