the presence of a film that is appreciably greater than that left 

 by the pure water. The water squeezed from the sponge should not 

 leave a sticky feeling on the fingers, nor should there be any ap- 

 preciable smell. 



Density Test 



The density of the spongin or structural material of sponges 

 was difficult to determine. No matter how finely the samples were 

 divided, they tended still to hold sand particles, which increased 

 their weight, or to hold bubbles, which decreased their weight. Best 

 results were obtained by cutting the sponge into thin slivers, pound- 

 ing and rolling the slivers between a glass rod and plate in water 

 containing Dreft until the sand was washed out, bringing a Dreft solu- 

 tion suspension of sponge material to a boil (with constant prodding) 

 to remove air bubbles, cooling and examining with a good lens to deter- 

 mine whether further cleaning was needed. The sponge material should 

 not be allowed to drain out any water until the density is determined 

 by displacement in the customary specific-gravity bottles, or the boil- 

 ing operation will have to be repeated to remove air bubbles. The 

 best tests indicated that the basic material of these sponges had a 

 density of l o 5>0 grams per cubic centimeter of sponge material. 



A more convenient figure, and one that does show some differ- 

 ence between types of sponges and individual sponges, is that of 

 the bulk density. This value is the weight of the sponge converted 

 to grams per cubic centimeter or pounds per cubic foot for the bulk 

 of the sponge. Where cellulose sponges have been manufactured with 

 an unusually low bulk density of 3»26 pounds per cubic foot, and a 

 urethane type sponge possessed a bulk density of 2»60 pounds per 

 cubic foot, the natural sponges treated were of even lower bulk dens- 

 ity of about 2 pounds per cubic foot. Not enough figures were ob- 

 tained to report reliable average values for other than the Rock Island 

 sponge, 



51 



