The apparatus for the production of the acid solution consists of the sulphur-burner O, the wash-vessel for the gas W, and the vessel T for dissolving the gas in water.
The sulphur-burner O is a small brick vault of sufficient size to hold a vessel S having a capacity of a few quarts. In front the sulphur-burner is provided with a well-fitting door J, which is furnished with a small aperture for the introduction of an iron pipe into the burner. The lead-pipe R leads from O to the bottom of the wash-vessel W, and from the lid of the latter rises a pipe R1, and runs along the bottom of the trough filled with water. This trough is provided with a wooden lid and the cock H, and by means of the latter the fluid can be discharged into the vessel G.
Fig. 58.
The trough T is filled with water and W is also filled three-quarters full. In the burner O is placed a dish S filled with sulphur, and the latter ignited. The door is then closed and air blown in through A by means of a bellows, the joints of the door J being at the same time luted with clay.
In contact with air, the sulphur burns to sulphur dioxide. The latter is freed in the wash-vessel from sulphur vapor which has been carried along, and passes from R1 through the numerous perforations into the water in T where it is dissolved to sulphurous acid.
Saturation with sulphurous acid of the fluid in the trough is complete when the suffocating odor of the acid is perceptible in the proximity of T. The fluid is then discharged, replaced by water, which is again saturated with sulphuric acid, and so on.
The cakes of glue to be bleached are placed in a trough (Fig. 58), in which are arranged several frames, B, covered with linen. The cakes of glue are placed upon these frames and the trough is filled with sulphurous acid so that it stands a few inches deep over the uppermost frame. The cakes of glue swell up rapidly in the solution of sulphurous acid, and yielding up their salts become bleached. After twelve hours the fluid is discharged through the cock, H, and if glue of a particularly fine appearance is to be produced the cakes are treated twice more with solution of sulphurous acid.
When bleaching is finished the trough is filled with clean water, in which the glue is allowed to remain for a few hours, when the frames are lifted out and the cakes dried.
By this method glue may be bleached to such an extent as to render it fit as a substitute for gelatine for many purposes, for instance, for the imitation of thin plates of ivory.