380 Scientific Proceedings, Royal Dublin Society. 



felspar (sp. gr. 2'58) were next added; tlie quartz sank to a level 

 below the spicules, the felspar remained above. As the contents 

 of the tube could be easily examined under the microscope with a 

 1-inch or even a 5-inch lens (Zeiss's 0), one could make certain of 

 the absence of air bubbles, vacuoles, or other troubles ; and as the 

 spicules could be seen individually, it was possible to determine the 

 specific gravity of a single one. The spicules did not all lie at 

 exactly the same level, but formed a zone thickest towards the 

 middle, and thinning off above and below ; a few stragglers were 

 seen at some distance on either side, but this was owing either to 

 adhesion to the side of the tube, or attached impurities. 



The specific gravity could now be exactly determined. Two 

 rectangular axes are ruled, fig. 2 ; on one distances are taken to 

 represent the densities of the calcite, quartz, and felspar; on the 

 other the exact distances between the middle line of each fragment 

 as. it floats in the tube are measured off. 



These distances were obtained by gumming two scales divided 

 into millimeters on the stage of the microscope, at right angles to 

 the glass slide carrying the experimental tube, i. e. parallel to this 

 tube (fig. 1, Sci and 8cn). The calcite was brought into focus, and 

 the position of one edge of the glass slide read off on the scales ; 

 the slide was then moved down till the quartz came into view, and 

 the position of the slide again read off on the scales. The object 

 of having two scales is obviously to ensure parallelism in the move- 

 ments of the glass slide. 



The specific gravities and distances being indicated on the rect- 

 angular axes, one constructs a curve which gives the change in 

 density from one mineral to another in the tube. 



The height of the zone of spicules being now indicated on the 

 axis of distances, a line is drawn parallel to the other axis through 

 it ; from the point where it cuts the curve a perpendicular is let 

 fall on the axis of specific gravities, and the point where it meets 

 the axis gives the specific gravity. In this way the specific gravity 

 of the spicules was determined to be from 2'61 to 2'63. They 

 are plainly, therefore, not aragonite, and, arguing from the specific 

 gravity alone, probably consist of calcite. The slight difference 

 between it and them in specific gravity is no doubt due to the pre- 

 sence of organic matter ; for within, a minute canal, filled with some 

 kind of organic material, possibly spongin, occurs in the axis of the 



