Mortar, thermostatic

The furnace and thermostatic mortar. For heating the tube packing, a small electric furnace N has been found to be more satisfactory than a row of gas burners. The type used consists of a silica tube (I s cm. in diameter and 25 cm. long) wound with nichrome wire and contained in an asbestos cylinder, the annular space being lagged the ends of the asbestos cylinder being closed by asbestos semi-circles built round the porcelain furnace tube. The furnace is controlled by a Simmerstat that has been calibrated at 680 against a bimetal pyrometer, and the furnace temperature is checked by this method from time to time. The furnace is equipped with a small steel bar attached to the asbestos and is thus mounted on an ordinary laboratory stand the Simmerstat may then be placed immediately underneath it on the baseplate of this stand, or alternatively the furnace may be built on to the top of the Simmerstat box. 

The thermostatic mortar P, whose function is described below, is a small electrical heating unit (1 5 cm. in diameter and 7 cm. long) kept constant at 180 . The temperature is kept constant by another Simmerstat. The mortal may be supported on its Simmerstat box or alternatively screwed on to the end of the furnace, a gap of 1 cm. being left between the furnace and the mortar in each case. The right-hand end of the mortar bore is only wide enough to take the drawn-out beak end of the combustion tube, which is thus held in place. 

The oxygen is turned on, tap Tj carefully opened and the furnace N and the thermostatic mortar P then turned on. The apparatus can conveniently be allowed to attain its equilibrium temperature over the period of about 20 minutes while the absorption tubes are being polished and weighed and the specimen for andysis weighed out it is thus ready for immediate use as soon as the weighings have been completed. 

At the end of the sweeping out, the tap Ti is first closed, and then the taps T3, T4, Ts and Tj in this order. The tubes R and S are then detached from the beak of the combustion tube, the guard tube V is then detached from them and replaced on the combustion tube beak. The furnace and thermostatic mortar are then switched off and the combustion tube allowed to cool with the tap to the oxygen supply open. The bung J is removed, and the boat withdrawn by means of a piece of rigid copper wire with a small hook in the end that fits into the small hole in the lip at the back of the boat the bung is then replaced and the boat transferred to its block in the desiccator. 

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