Bes 
The acids from munition works have attracted attention of late. An 
effluent composed of 0.13 to 0.4 per cent. of acid—a mixture of 2 parts of 
sulfuric acid and 1 part of nitric acid—is discharged by guncotton works. 
This acid effluent flowing into the brackish waters of the coast of New 
Jersey repelled the killifishes, on which the keeping down of mosquitoes 
depends. It was proposed to treat the acid effluent with lime, and the 
question of the effect of the calcium nitrate on the fishes became a prob- 
lem for immediate solution.* Similar problems are arising in connection 
with inland rivers. Large quantities of such acid is now being run into 
the Sangamon River by munition works at Springfield, Ill., and into 
various other waters of this state. 
The Gradient Tank—The gradient tank in which experiments 
designed to answer question V are performed, should be twice as wide, 
three times as deep and twenty times as long as the length of the fishes 
to be tested (Shelford and Allee, 13; Powers, ’14; Wells, ’15). Where 
the density of the water introduced at the two ends is different inclined 
screens should be used (Shelford and Powers, ’15, p. 327). 
Temperature —It is necessary to see that the temperature of the 
water at the two ends of the tank does not differ more than 0.2°C. (Wells, 
14; Shelford and Powers, ’15). 
VI. Do polluting substances cover the bottom and make conditions 
unfavorable for eggs? 
The majority of important fresh-water animals—mussels, which 
furnish pearl for buttons, whitefish, bass, sunfish, ete—are dependent on 
the bottom for breeding, living conditions, or food. For example, bare 
clean terrigenous bottom is necessary as a resting-place for eggs of the 
whitefish of the Great Lakes (Clark,’10). It breeds in 8-25 meters of 
water. Many of the important breeding-grounds in the Great Lakes have 
been ruined, or at least rendered unusuable for a long time to come, by 
the accumulation of slowly putrescing sawdust, water-logged wood, etc. 
(Clark, ’10). In Grand Traverse Bay, Lake Michigan, in 1871, salmon 
ova were discovered to be diseased and decaying, with particles of saw- 
dust adhering to them (Milner,’74). Tarry material often covers bot- 
toms and destroys life there. In the case of marine animals such as 
herring, clams, and mussels, similar serious results may occur. 
Bottom-sampling—The Petersen bottom-sampler is the best instru- 
ment to be used in deep water (Murray and Hjort,’12, p.785). For 
shallow-water methods see Baker, ’16. If the contaminating substances 
are covering breeding bottoms of bare sand and gravel they are danger- 
ous (Knight, ’03). f 
Bottom Index Organisms—The presence of gilled snails of the 
genera Pleurocera and Goniobasis in fresh water indicates clean bottoms. 
Various other organisms usually indicate pollution with sewage (Forbes 
and Richardson, 13). 
*¥or this information the writer is indebted to R. S. Patterson, of the New 
Jersey Mosquito Commission. 
