SODIUM BISULPHITE.--A formula for use of this chemical is given in Chapter VIII., and is applicable to latex standardised to 1-1/2 lbs. dry rubber per gallon. If a higher standard is chosen the quantity calculated as in (_b_) of that formula may be increased slightly, and the exact requirements found by experience. The caution must again be given that the employment of an excess of sodium bisulphite will lead to the production of an over-pale rubber, and a prolongation of the drying period. If thick crepes are made, an excess of the chemical is sometimes made visible by a greyish powder deposited on the edges of the strips of dry rubber. It must be emphasised that the formula in Chapter VIII. indicates the _maximum_ quantities required for use with standard latex. Many estates will find it expedient to use less of the chemical; and if it is found that the desired result is not obtained from normal proportions, attention should be directed to the points discussed in the following paragraph. EVALUATION AND DETERIORATION OF SODIUM BISULPHITE AND SODIUM SULPHITE.--Sodium bisulphite and sodium sulphite are both bought for our purpose in the form of a fine crystalline powder, and on analysis good specimens should contain over 90 per cent. pure substance, when packed in well-sealed vessels. It has often happened that shippers or local sellers, by inadvertence, have supplied the one chemical in place of the other--to the detriment of the rubber and the discomfiture of managers of estates. The error, as a rule, has not been detected for some time, and then perhaps only as a result of complaints or enquiries reaching the laboratories. To the layman, and certainly to the native who usually has charge of these substances, it is not a simple matter to distinguish between them without special knowledge. There are certain elementary tests, however, which can be applied on all estates serving to make the distinction, but affording no information regarding the actual quality of the chemicals. They are given in a comparative form on page 116. Samples of doubtful specimens may be sent to the laboratories for analysis, but the bulk of the chemical should not be used. During the War some very poor shipments were received, and much trouble was caused. Under normal conditions there can be no question that it is to the interests of chemical manufacturers to supply the best article; and it is anticipated that in future there should be no difficulty in procuring shipments of a high degree of purity.