218 VICTOR E. SHELFORD AND W. C. ALLEE 



The analysis shows that the differences between the mineral 

 content of tap water and the water that passed through the 

 apparatus are very shght. No traces of the various metals used 

 in the construction of the apparatus appear in the water. There 

 is a loss of 4.8 parts per million of calcium. Iron, magnesium 

 and sulphuric acid were slightly increased. The absence of any 

 traces of the metals contained in the apparatus is largely due to 

 the fact that they quickly became covered with sediment and 

 scale and the water did not actually come in contact with them. 

 The boilers are of aluminum which also becomes covered over 

 with aluminum oxide which is insoluble in water and only slightly 

 soluble in acids. Aluminum is probably the best metal from 

 which to make apparatus for biological purposes, principally for 

 this reason. 



The Chicago tap water usually contains quantities of algae, 

 some rotifers, Protozoa and Entomostraca, the last usually dead. 

 The boiling process kills and cooks any plankton in the water 

 so that plankton-feeding fish might be able to secure food from 

 the tap water but not from the boiled. This, however, is prob- 

 ably of no importance in the cases of the fish used in these experi- 

 ments (see p. 220). • 



(2) Introduction of gases. Gases are introduced from tanks 

 between the upper and lower coolers and by means of the gas 

 introducer (fig. 1 A). The tank of gas is attached to the intro- 

 ducer by means of a rubber hose. The gas enters the chamber 

 (ch) and passes through a number of the small openings. Large 

 quantities of oxygen, carbon dioxide, or other soluble gases can 

 thus be added. If it is desired to add a soluble gas directly to 

 the tap water j valve G is opened and valve F is closed, thus 

 allowing the water to flow directly into the delivery tank. It 

 is necessary, when untreated tap water is used, to allow the gas 

 to enter only in such quantities as will go into solution in the 

 cooler coil, for if it comes off in bubbles other gases are also 

 removed. When gases are to be added to water that has been 

 boiled, but little gas is removed by any bubbles which may pass 

 through, and here it is sometimes desirable to allow an excess to 

 escape as bubbles by inserting a tee with the stem projecting 

 upward between the rubber hose and the withdrawal cock. 



