vironment so that different plants and animals can be observed during several generations at specified and perfectly controlled temperatures. Four rooms are kept at + 5° C, + 10° C, + 15° C. and + 20° C. The carbonic acid machine (Fig. 14) produces 5,500 calories hourly.

The fluid carbonic acid flows into a wrought-iron spiral core within a carefully isolated receptacle made of tin plate. The salt water surrounding the spiral core is cooled to a low temperature by the sudden

Fig. 13. Warm Room with Compartment for Elevated Temperature on the Right.

expansion of the liquid carbon dioxid. Then the carbonic acid gas is compressed under 55 atmospheres and conducted into a wrought-iron spiral core in the condenser where it is again cooled, liquefied and ready to flow on in the continuous circulation through the machine.