( 27 ) 



accurate we used, therefore, an Arbf., tlic more so as it turned out 

 to be suftieiently accurate for our purposes. 



Among the thuds, not mixable with water, we took advantage of 

 the series Hquid paratfm (?i = 1.4759), petroleum (/^ =: 1.45(j8), 

 benzin (;i = 1.3994) and petroleum-ether (m=: 1.3780). 



We succeeded by using mixtures of petroleum and benzin in fixing 

 the refractive index of the spicules at 1.4508 — 1.4510. In order to 

 give an idea of the degree of accuracv that can be attained in this 

 way, we may state tiiat undoubtedly a difference is to be seen between 

 spicules, mounted in a mixture of 20 cc. of petroleum with 3 cc. 

 of benzin (y^^ 1.4500) and mounted in a mixture of 20 cc. of petro- 

 leum witii 2.5 cc. of benzin (ii = 1.4510). Fora aqueous watery solulion 

 we used dilute glycerine, and found with this medium also a com- 

 plete disappearance at ?i=: 1.4508. In addition to these media we 

 studied the influence of air, methylic alcohol, water, potassium acetale, 

 creosote, oil of bergamot, Venetian turpentine, oil of red cedar wood, 

 oil of lemon, oil of thyme, firoil, oil of peppermint, oil of cloves 

 (pure or mixed with alcohol), Canada balsam and m()nol)rom- 

 naphtaline. Practically however, we used more especially the two 

 fluids mentioned above. 



In glycerine with y< =r= 1.4508 indeed the spicopal of some spicules 

 disappears completely, and only the axial rod remains visible as a 

 light bluish thread. In all styli, the central thread has a higlier 

 refractive index than the spicopal. By careful examination (Zeiss 

 Apoclir. 8.0c. 4) in most spicules a light, sharp line may be seen 

 as border, and a system of longitudinal striae between the axial rod 

 and the border. With low power the lamellar structure does not 

 become conspicuous, though the borderline is still visible. Most 

 probably this thin line, which exhibits double contours with high 

 power, I'epresents the organic spicule sheath. 



By these experiments it becomes at the same time very evident, 

 that the axial thread can be partly al^sent; on these spots the 

 light bluish band (the central rod) abruptly ceased. If the glycerine 

 has entered into the central canal, only an indication of the spicopal 

 is visible ; if air has entered, of course this is directly visible 

 by the lower refraction. The aspect of styli isolated by means of 

 boiling with hydrochloric acid, either with addition of potassium 

 chlorate or without, or l)y digestion by means of artificial gastric 

 juice, or by heating with sulphuric acid and potassium-bichromate, 

 fundamentally agree. 



Quite another aspect is shown by spicules which iiave been dried 

 for some days in the presence of anhydrous phosphoric acid. We have 



