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limiting plains. We hope to show that the well-known stripes are 

 nothing but the optic sections of such limiting plains, and that thej 

 are independent of eventual differences of the layers. It is easy 

 enough to microscopically demonstrate sucii limiting plains in layers 

 in artiticial siliceous gels. If a coat of not yet coagulated siliceous 

 gel is poured out over another one freshly coagulated, and if this 

 is repeated, it becomes evident that the consecutive layers of gel in 

 the beginning do not unite. Only in drying the layers become one 

 mass; still, in section the limiting plains are very conspicuous. This 

 experiment teaches us that in a siliceous gel a lamellar structure 

 can appear, wherein the consecutive layers are separated by visible 

 limits, without interference of another substance e. g. of an organic 

 lamella. We will come back to this fact later. 



Several ways are open to us for the study of the spicular structure, 

 but the}^ are not easy. Besides dissolving the spicopal by means of 

 hydrotluoric acid and carefully watching the process, the method of 

 heating has been applied since Gray (1835) showed that this brings 

 out more distinctly the lamellar structure. Ijima (1901) was the first 

 to point out the eifect of ditferent media. We shall see, that some 

 spicules mounted either in Canada balsam or in glycerine, so widely 

 dilfer in aspect that on first sight one believes that one has to deal with 

 entirely different sorts of 8[)icules. We thought it necessary, therefore, 

 to begin by determining the refractive index of spicopal of various 

 spicules somewhat more accurately than hitherto done. As far as 

 we know of there exists no other information than given by Sollas 

 (1885), who states in general that 'ihe refractive index of sponge- 

 silica is ... . that of opal or colloidal silica, and not of quartz", 

 and that the spicules come nearest to invisibility when "mounted 

 in chloroform, which possesses a refractive index of 1.449". 



In determining the refractive index of the s})icules we used the 

 method, since Sollas generally used also in mineralogy, viz. to find 

 in what tluid the spicule can no longer be seen. Perhaps there is a 

 still better criterion to make out how much a spicule differs from 

 its medium and in which direction, viz. the appearance of coloured 

 borders. — In order to avoid the effect of fluids wich might influ- 

 ence the amount of water contained in the spicopal, but, on the 

 other hand to demonstrate just this influence, we used fluids which do 

 not mix with water as well as such which were diluted with water. 

 The refractive nidices were determined by the refractometer of Abbe, 

 which has the great ad\'antage of enabling us to work with ordinary 

 dayligiit and to determine any number of indices, between the micros- 

 copical work. In spite of the apparatus of Pulfrich being more 



