These always contain, besides powdered dried soap, a large percentage of sodium carbonate, generally in the form of dried soda crystals. They may be prepared in several ways, thus: -

(a) Anhydrous sodium carbonate or soda ash is added to a "clear-boiled" soap-paste, and after thorough mixing, the somewhat stiff material is drawn off into cooling frames. The cold and hard soap thus obtained is then finely ground.

(6) Soda crystals and soap are melted together and then treated in the above manner; but this plan is advantageous only when soap scraps can be had.

A suitable apparatus for conducting the operation consists of a wrought-iron vessel with a strong agitator contained in an interior cast-iron vessel, which can be cooled by water circulated in the outer vessel. The liquid soap is cooled while the soda ash is slowly added and completely dissolved. During the grinding process care must be taken not to overheat and thus soften the product.

The composition of soap powders varies considerably. Only a small proportion of rosin soap can be used, as such soap is sticky, and cannot be powdered. Olein soap may be used with advantage, and the olein may be saponified with sodium carbonate instead of the more expensive caustic lyes.

As a small quantity of free chlorine is not objectionable in soap powders, dark coloured materials such as bone fat, fish oils, etc, may be used for making the soap, with the addition of a small quantity of bleaching powder.

To some soap powders, 2 to 5 per cent. of sodium silicate is added.

A good washing powder should contain 30-35 per cent. fatty acids, 30-35 per cent. sodium carbonate, 30-40 per cent. water.

The inferior powders, containing only 5-10 per cent. fatty acids, should not . be used for the laundry; they are only serviceable for scrubbing purposes. There is a soap powder in the market prepared by treating linseed with caustic soda directly. This soap contains certain impurities derived from the seeds, which lather freely, and thus, when the powder is used, gives the impression of more genuine soap being contained in the powder than is actually the case.

For washing printers' blocks and types, a powder is recommended containing only a small quantity of combined fatty acids, but 10-15 per cent. caustic soda.

The so-called "bleaching soda" consists of 80 parts soda crystals and 20 parts soda silicate.

The composition of some American washing powders was found to be -

a

b

C

d

Sodium carbonate

45.2

26.9

49.2

46.6

Fatty acids.. ..

26.4

44.0

25.6

25.7

Combined Soda ..

3.1

3-4

3.5

2.6

Water......

23.7

8.8

19.1

24 9

Fl e sand.....

"

16-3

"

"

They are therefore mixtures of dried soap and washing soda, and may be used in conjunction with soap to soften hard water, but if used alone must be injurious to textile fabrics. Borax might be added instead of the soda, apparently with advantage.