THE REGULATION OF TEMPERATURE. 



67 



temperature of the hydrant water and that which is to be maintained 

 in the bath. If the hydrant water is to be cooled before entering the 

 bath, as when a low temperature, e. g., 5°, is to be maintained in sum- 

 mer, it is first passed through the coils of pipe seen in Figure 33, which 

 are embedded in ice. The arrangement shown in Figure 32 C is also 

 employed in this bath to secure a constant pressure upon the circulating 

 water. 



Fig. 34. — Rectangular bath for general laboratory use. 



(1) to (8) Brass tubes for circulation of hydrant water; (9) and (10) copper cylinders, opening on 

 opposite sides of the bath, for the lamps; (11) pump; (12) and (13) pipes through which water 

 is drawn out of the bath and over the gas stoves seen at the end; (14) large pipe through 

 which water heated by the gas stoves is drawn and delivered at (11). 



A word of caution may be given regarding the valves to be used when 

 a constant pressure on running water is to be maintained. Our first 

 pressure arrangements were constructed in accordance with correct prin- 

 ciples, so far as we knew, but it was found that they would not maintain 

 constant pressures. The flow of water diminished continually, and 

 very small streams ceased altogether after a time. After a long search, 

 the difficulty was located in the valves. Those we were using — the 

 so-called "gate-valves" — were found to be so constructed as to permit 

 the accumulation of the gas which is expelled from water, when its tem- 

 perature is raised, to such an extent as to impede the flow of the water, 

 and to stop it altogether if only a little were passing through the valves. 

 After replacing the "gate- valves" by others of the common lever 

 variety, the difficulty disappeared. 



The "heating" surface is furnished by the two copper cylinders (9 and 

 10), the latter of which is broken in order to show the location of the 

 stoves. The large wooden box is lined with copper, and the two copper 

 cylinders in question extend entirely through it from side to side, and are 

 opened at both ends upon the outside of the bath. They are closed with 

 caps, upon the inside of which are fastened the lamps. Provision is 

 thus made for four lamps which are usually of 16 candle-power, though 

 lamps of 8 candle-power often suffice at low temperatures. The lamps 

 (stoves) are regulated according to the scheme already explained. 



