and that claimed, viz., 4.30 per cent. Na2O, is due to the 7.35 per cent. Na,CO,, this being estimated as its equivalent, 4.30 per cent. Na2O, in the total sodium oxide. As more or less carbonate is invariably present in all commercial caustic, especially in the lower grades, the system of including it in the expression of the quality of this product is open to severe criticism. Quotations of quality are thus confessedly a misrepresentation. The only rational method is the expression of the Na2O as free caustic, or preferably units of NaOH.

This would be an absolute index of the value of the caustic as a saponifying agent, and not, as by the method in vogue, an uncertain. approximation of the same. English degrees indicate the strength of the ash or caustic in terms of Na2O, but, owing to either a wilful or accidental error in atomic weights, English analyses indicate a greater percentage of Na,O than is actually present. This error has become so firmly established by tradition that modern ideas have been unable as yet to eliminate it. In Germany and Russia the strength is expressed in terms of sodium carbonate. This system is perfectly rational when applied to soda-ash, but is inconsistent when applied to caustic. The expression of the value of commercial caustic soda in terms of an impurity, which, in so far as the scap industry is concerned, is positively worthless as a saponifying agent for neutral glycerides, is certainly not conducive to clear ideas on the subject.

The superior advantages and economy of high-grade caustic need no argument. It is true of this product that the best within certain limits is the cheapest. There is presented in the following table the price per pound of sodium hydrate as it occurs in the customary grades of caustic, assuming that no carbonate is estimated as caustic:

The anomalous variation in prices quoted arises from the slight differences in cost of production of the lower grades, combined, for those grades, with the proportionally greater cost of packages, transportation, etc.

The total charges contingent upon marketing a 60 per cent. caustic are the same as those of a 70 per cent., although the former contains considerably less of the essential ingredient; also the cost of production of a 70 per cent. caustic is but little more than that of 60 per cent. The increased cost of production of the higher grades, viz., 74 per cent. and 76 per cent., makes necessary a higher price, which is less than it would be if cost of marketing were correspondingly increased.

The system of quoting the higher grades in terms of 60 per cent. caustic is a peculiarity of the trade, and the fractional increase in price based upon the degree is a measure of the increased cost of carrying the manufacture of the product above the 60 per cent. grade.

In the preparation of caustic lyes of different densities from various grades of caustic, the influence of the impurities, chiefly sodium chloride, sodium carbonate, and sodium sulphate, is to reduce the active value of the solution for the specific gravity indicated. This reduction in saponifying power is least for the highest grades and greatest for the lowest as a natural result of the increased percentage of these impurities present. The following table indicates the percentage of sodium hydrate present in lyes of different densities, made of the usual grades of caustic, corresponding to the densities of lye made from chemically pure caustic. It is assumed that the total alkali is present entirely as caustic, which never actually occurs. The figures stated, for reasons previously given, are generally slightly higher than would be found in practice. However, the table possesses value as a basis of comparison, and for all technical purposes the figures are sufficiently accurate.

Causticization of Soda-ash.-The consumption of caustic soda for oil-refining and soap-making may be of sufficient amount, together with other economic considerations, to warrant the manufacturer undertaking the manufacture of caustic soda himself. Caustic soda in aqueous solution is obtained from soda-ash, i.e.,

TABLE 9.-PERCENTAGE OF SODIUM HYDRATE IN LYES MADE FROM VARIOUS GRADES OF COMMERCIAL CAUSTIC.

1.308 34 27.80 27.26 26.55

26.83 26.31

25.62

24.92

24.23 20.77

59.4

25.82

25.11

1.320 35 28.83 28.28 1.332 36 29.93 29.35 27.81 1.345 37 31.22 30.62 29.82 29.00 28.46 24.40 69.0 32.47 31.84 30.99 30.16 29.32 25.13 71.4

21.52

61.6

27.53

26.79

26.04 22.31

64.0

28.58

27.04 23.17

66.4

1.357 38

commercial sodium carbonate, by treating the latter, under suitable conditions, with slaked lime. The procedure of manufacture is in accordance with the following chemical reaction:

Na,CO,(soda-ash) +Ca(OH)2(slaked lime)

Thus, theoretically, 106 parts of sodium carbonate treated with 74 parts of calcium hydrate yield 100 parts of calcium carbonate and 80 parts of sodium hydrate. On a practical scale the theoretical yield is only approximately attained.

Description of Plant.-In Fig. 50 is shown, in plan and elevation, the mechanical equipment of a causticizing plant, consisting of the converting-kettle, screen-tank, lye storage-tank, evaporator, pumps, etc. Fig. 49 is a conventional form of lime-house. Fig. 51

is an enlarged sectional view of the screen-tank; and Fig. 52 is an enlarged sectional view of the lower portion of the converting-kettle.

It is best to provide a building or a suitable fire-proof room or inclosure for the storage and preparation of the lime to be used in the converting-kettles, and as imperfectly burnt or inferior qualities of lime often contain stone or unburnt material, such is preferably crushed before being passed to the converting-kettle, as it also affords an easier, more rapid, and complete operation in the converting-kettle.

In Fig. 49, I represents the lime-house, having a raised or elevated iron floor 2 for the storage of the lime. Adjacent to one side of this elevated floor is a door 3, through which the lime can be received.

4 is a lime-mill or crusher operated by a suitable power, a counter-shaft 5 being shown. The crushed lime from the mill may be passed directly into the carrier 7, which may be of perforated iron if it is to be used as a basket inside of the converting-kettles,