88 An improvement over the original system of Pettehkofer and Voit is a mercury valve which checks the rate of working of the valve. 3y means of a screw at the bottom, the level of the mercury can be rasied somewhat and in this way a greater resistance to the passage of the air current made. These screws and the valve arrangement are shown in Pig. 42. A small sketch of this regulator is given in Pig. C. The small mercury pump, with the Voit valve used for carrying the air current through the small sampling meters, may be seen in Pig. 43, at the extreme right of the mercury valve regulator just mentioned. An ingenious clamp is also used by Voit to hold the barium-hydroxide tubes, and if one were to employ this method, Voit's new clamp should certainly be used. One of the chambers is completely surrounded by a water mantle (see Pigs. 44 and 45) so that experiments can be made at different temperatures, a petro¬ leum regulator in the bath regulating the heating of the water. The same general type of petroleum regulator is used in other places in the labora¬ tory, as, for instance, for holding a bath at a constant temperature of 15°, for keeping the water at a constant temperature in calibrating burettes, and for keeping standard solutions at a constant temperature. This use of a con¬ stant temperature is typical of many of Erwin Voit's methods for recording temperatures. He is extremely accurate and extremely precise. The gas meters are calibrated by means of water, before and after each experiment, and as previously stated, Professor Voit finds them extremely accurate. As a matter of fact, in analyzing the ingoing air, room air is used and Professor Volt sees no reason to think that it might be better to use outdoor air. These gas meters are calibrated and a description of the method of calibrating was given by Voit is a paper in the Zeitschrift f. Biologie, 3d. XXII, p. 281, entitled "Ueber die Aichung der Gasuhren". Pro¬ fessor Voit has sent us a copy of this. Laboratory construction.- I found much to interest me in the laboratory, the construction of the hoods being very good indeed. Structural steel is