SoLLAs — On Sponge Spicules. 375 



waters, and consists of silica (probably one of tbe silicic acids) in 

 intimate combination witb organic matter." ^ 



More recently M. Thoulet,^ to whom apparently my results 

 were unknown, has made some experiments which completely 

 confirm the preceding statements, except with regard to the 

 presence of organic matter in the sponge-silica. M. Thonlet 

 found that by calcination certain sponge spicules which he ex- 

 amined lost 13*18 per cent, in weight; their specific gravity was 

 determined to be 2*0 361 at 16-5° C. ; and hence M. Thoulet con- 

 cludes they are to be regarded as opal, which is only another name 

 for consolidated colloid silica. My attention having been re- 

 directed to this subject by M. Thoulet' s observations I wished to 

 determine exactly the refractive index of sponge-silica, but was 

 met at once with a difficulty due to the form of the spicules, their 

 shafts being more or less cylindrical, so that their surface is 

 always strongly curved, never plane. It then occurred to me to 

 take advantage of the fact that a solid body becomes invisible, or 

 nearly so, when immersed in a liquid having the same refractive 

 index as itself. The problem then was to find a fluid in which the 

 sponge spicules could no longer be seen. Knowing the refractive 

 index of this liquid, the refractive index of the spicules would also 

 be known. Experimenting with various kinds of sponge spicules, 

 including fragments of Lithistid skeletons, I found that the nearest 

 approximation to invisibility, as seen under the microscope, was 

 obtained when they were viewed mounted in chloroform, which 

 possesses a refractive index of 1-449. This is very nearly the same 

 as that of iridescent opal (1"446). Some colloidal silica, which I 

 prepared some five years ago, and which has been hardening ever 

 since, gave a refractive index ofi|l'47, being least visible when 

 mounted in glycerine. 



The refractive index of sponge-silica is therefore that of opal 

 or colloidal silica, and not of quartz, and thus my earlier state- 

 ments, and those later of M. Thoulet are confirmed. All difficulty, 

 therefore, with respect to the substitution of ealcite for silica in 



1 Sollas " On Pharetrospongia Strahani ; " Quart. Journ. Geol. Soc, vol.xxxiii. p. 

 254, 1877. 



- Thoulet.—" Siir Ics spicules siliceux des eponges vivant." Compt. Rendu, xcviii. 

 pp. 1000-1. 



