SoLLAs — On Sponge Spicules. 



379 



lecting tube, fig. 1, T, about 2 inches long, and f-incli in diameter, 

 was cemented with plenty of Canada balsam to a glass slide, as in 

 the illustration. 



Pig. 1. 



Arrangement for determining the density of minute objects under the microscope. S, stage of 

 the microscope ; Sc-^ and Sc^, the two scales ; G, glass slide carrying the observation 

 tube, T. 



The object of using excess of balsam was to destroy optically 

 the curvature of the side of the bottle. As the refractive indexes 

 of Sonstadt's solution and balsam are not very different, this 

 plan succeeded admirably. A thin cover-glass was similarly 

 cemented to the opposite side (front face) of the bottle, which was 

 thus optically flattened, front and back. Some Sonstadt's solu- 

 tion (sp. gr. 2-77) being introduced, a fragment of aragonite (sp. 

 gr. 2 '9} was dropped in ; it at once, of course, sank to the bottom. 

 Next, a piece of calcite (sp. gr. 2-7) was added ; it floated on the 

 surface. The spicules, lying in water, were freed as far as possible 

 from air by boiling, and with the air-pump. With a dipping-tube 

 the water and spicules together were taken up and added to the 

 top of the Sonstadt's solution, where they floated. The tube was 

 then left to stand in order that diffusion might take place. After 

 some hours the water and Sonstadt's solution had become gradually 

 mixed, giving a column of fluid with a specific gravity of about 2 "4 

 at the top, and 2*77 at the bottom. The calcite and the spicules 

 floated at different levels (the spicules being above), in layers of 

 fluid having respectively the same specific gravity as themselves. 

 A fragment of pure quartz (sp. gr, 2'65), and another of adularia 



2 K 2 



