12⁄12ths of 1°. They are then well corked up, and, after agitation, arranged side by side on a tray, furnished with holes to receive them. (See engr.) To ascertain the strength of an unknown sample of bleaching liquor, the proof-solution of iron is put into a phial, exactly similar to the 12 previously used, and in precisely the same proportion (1⁄9th). The phial is then filled up with the bleaching liquor, well shaken, and placed beside that one of the 12 already prepared which it most resembles in colour. The number on that phial expresses the strength of the sample under examination, in twelfths of a degree of Twaddell’s hydrometer.
Obs. The preceding method is admirably suited for weak solutions, such as are employed for bleaching textile fabrics, and is well adapted (from its simplicity) to the purposes of practical men. Indeed, it is quite astonishing to see with what ease and accuracy it is applied by unlettered operatives. This gives it great practical value. It has been for some time in extensive use in the bleaching houses of Scotland.
Table exhibiting the quantity of Bleaching Liquid, at 6° on Twaddell’s scale (sp. gr. 1·030), required to be added to a weaker liquor, to raise it to the given strengths. Adapted from Mr Crum’s table by Mr Cooley.
| Strength of Sample in 1⁄12°. | Required Strength. | Proportions required. | |
| Given Sample. | Liquor at 6°. | ||
| Parts. | Part. | ||
| Water. | 8⁄12° | 8 | 1 |
| 1 | ” | 91⁄4 | 1 |
| 2 | ” | 11 | 1 |
| 3 | ” | 131⁄2 | 1 |
| 4 | ” | 17 | 1 |
| 5 | ” | 23 | 1 |
| 6 | ” | 35 | 1 |
| 7 | ” | 71 | 1 |
| Water. | 6⁄12° | 11 | 1 |
| 1 | ” | 131⁄2 | 1 |
| 2 | ” | 17 | 1 |
| 3 | ” | 23 | 1 |
| 4 | ” | 35 | 1 |
| 5 | ” | 71 | 1 |
| Water. | 4⁄12° | 17 | 1 |
| 1 | ” | 23 | 1 |
| 2 | ” | 35 | 1 |
| 3 | ” | 71 | 1 |
| Water. | 3⁄12° | 23 | 1 |
| 1 | ” | 35 | 1 |
| 2 | ” | 71 | 1 |
According to Mr Crum, the range of strength within which cotton is “safe” is very limited. A solution at 1° of Twaddell’s scale (sp. gr. 1·005) is not more than safe, while one at 1⁄2° is scarcely sufficiently strong for the first operation on stout cloth, unless it is packed more loosely than usual.
Gay-Lussac’s Indigo Process. One part of the best indigo is dissolved in 9 parts of strong sulphuric acid by the aid of a gentle heat; this solution is then mixed with distilled water, in such proportion that 1 volume of chlorine gas shall exactly decolour 10 volumes of this solution. Each measure so decoloured is called a degree, and each degree is divided into fifths. 5 gr. of the best chloride of lime, dissolved in 500 gr. measures of water, possess the above power, and indicate 10° or proof; or in other words, will decolour 10 times its volume of the indigo solution.
Obs. This method of chlorometry is objectionable, and liable to error, from the indigo solution altering by keeping. When, however, the proper precautions are used, it may be safely trusted for weak bleaching liquors.
Arsenious Acid Process. This depends on the conversion, by oxidation, of arsenious acid into arsenic acid, in the presence of chlorine and water.
To prepare the test-liquor, pure arsenious acid, 100 gr., are dissolved in about 4 fl. oz. of pure hydrochloric acid (free from sulphurous acid), and the solution diluted with water until, on being poured into a graduated 10,000 grains-measure-glass, it occupies the volume of 7000 grains measure marked on the scale. Each 1000 grains measure of this liquid now contains 14·29 gr. of arsenious acid; corresponding to 10 gr. of chlorine, or 1⁄10th gr. of chlorine for every division or degree of the scale of the chlorometer.