ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 583 
tube d. The whole arrangement is fastened in the partition wall n 
which divides the front part of the hot-stage into two parts. The 
cylinder g is filled with air. As this air expands or contracts on heating 
or cooling, so does the piston & move forward or backwards. When 
the piston has moved forward a certain distance its plate p comes in 
Fig. 86. 
contact with the end of the exit tube d and closes it. In that case no 
water can flow out, and consequently no more hot water can enter. The 
apparatus then cools by radiation, the volume of air in g diminishes, 
the piston is drawn back, d opens and water again flows in until the 
piston once more closes d and so on. In actual practice d is never 
2 q 2 
