Mr. Macadam on the Paper Manufacture. 129 



in them. The process recommended was that in which the peroxidation of 

 the proto-sulphate of iron is made the index of strength. This process is 

 best suited for commercial purposes, being easy of performance and speedy 

 in result. It has long been a matter of difficulty with paper makers (as 

 well as others) to find material of which to construct vessels capable of 

 retaining solutions of bleaching powder for any length of time without 

 leakage, as also to form the necessary piping, &c. for transmitting the 

 liquor to the bleaching engine. This has been supplied in gutta percha. 

 The receiving vessels are built of wood, lined with this substance. This 

 application of gutta percha has been found not only in the highest degree 

 effective, but consequently economical, the casements having been in use 

 for fully twelve months, without being injured to any perceptible extent. 

 The boots of the workmen in this department, and the smaller vessels 

 employed are now manufactured from the same material. The hydro- 

 meter can be used with considerable accuracy in determining the 

 value of bleaching liquors, when the original bleaching powder has 

 been proved by experiment to be of good quality ; but this instrument 

 does not indicate the presence of impurity, and consequently the 

 proportion of available chlorine. This is especially true in determining 

 the value of partially exhausted liquors, or those which have been 

 already in use. The hydrometer in this department was therefore 

 superseded by the very satisfactory process of Mr. Walter Crum, as 

 detailed in the printed proceedings of this Society. This process depends 

 for its use upon the red colour of the per-acetate of iron, produced when 

 solutions containing chlorine are brought in contact with the proto-acetate 

 of iron ; the depth of tint depending solely on the amount of available 

 chlorine in the solution tested, irrespective of any generally occurring 

 impurities which may be present. As the waste bleaching solutions from 

 the pulp are collected, and form the basis of the contents of the bleaching 

 engine, much foreign matters are unavoidably present in them ; in short, 

 the products of their previous bleaching action. The hydrometer cannot 

 therefore be used as a test for the value of such partially spent liquors ; 

 thus, on testing a previously used solution of bleach, whose specific gravity 

 was If by Twaddell's hydrometer, this liquor gave a lighter tint, with the 

 stipulated proportion of " Crum's proof solution," than a fi'esh unused 

 liquor at ^ T. yielded. Another advantage attending the introduction of 

 this testing process, was the capability of having a liquor of constant 

 value in the cistern appropriated for the reception of the waste bleaching 

 solutions, -as well as a standard for fresh or unused ones. Moreover, as 

 the varieties of pulp obtained from dissimilar kinds of rags require bleach 

 of different strengths, the charging of the engine for each variety was 

 effected with t\\Q greatest regularity, by this constancy in the value of 

 the partially spent solutions, since the larger or smaller proportion of tlie 

 standard fresh liquid was easily added to the contents of the engine, 

 according to a scale of measures for each variety of rag. Phials indicating 

 the tints produced by the whole mass of bleaching liquor, suited for each 

 Vol. UL— No. 2. 5 



