STANDARDISING INSTRUMENTS.--For standardising latex by dilution all that is required is an instrument which will preserve a perpendicular position while floating in latex, will be sufficiently sensitive to indicate fairly small differences in density of latex, and has one mark on its aerial portion accurately indicating a density corresponding to the required standard. On scientific grounds it can be demonstrated that such an instrument as employed in common practice would not be strictly accurate.[6] It is not proposed, in this section of the book, to discuss such considerations. [6] De Vries, "Archief voor de Rubbercultuur." Instruments of this nature are represented by the "Metrolac" (originating from the Rubber Growers' Association) and other similar recorders. They generally consist of a submersible bulb with a projecting stem which is graduated. The "Metrolac" differs from others in that the bulb is of torpedo form (thus reducing "skin friction"), and the graduations on the stem indicate actual weight of dry rubber per gallon instead of the ordinary specific gravity figures. Theoretical considerations to the contrary, it is found in actual practice in Malaya and Ceylon that, although such instruments are naturally delicate and require careful manipulation, they are of considerable practical value and satisfy a definite requirement. Until an instrument of greater accuracy and equal simplicity can be discovered all estates should regard the possession of a few "Metrolacs" as essential. The nature of their construction and the average conditions under which they are used (and abused) make it impossible to rely upon their accuracy indefinitely or for any long period. It is always recommended, therefore, that there should be at least two instruments available, one of which may be in daily use, while the other is kept in safe custody and only employed, say, once a week for purposes of checking the accuracy or degree of inaccuracy of the other. This can be done with reasonable approximity by placing both instruments in a tall vessel containing well-mixed and diluted latex. Instruments showing a marked degree of inaccuracy should not be preserved; but in cases of necessity "Metrolacs" from estates belonging to company members of the Rubber Growers' Association may be sent to the laboratories for repair and adjustment.[7] [7] This applies to the gilt brass instruments. As the result of experiment the Rubber Growers' Association are now introducing glass instruments. These are necessarily more fragile, but while unbroken can be relied on to give correct readings.