WAX SIMULACRA OF SHELLS. 227 



kept running on to the mass already on the surface of the water, 

 otherwise a long irregular band is produced which has no 

 particular interest. I have not been able to make the wax flow 

 equally all round the umbo, to produce a resemblance to the 

 limpet shell, because such a mode of flow would be a case of 

 unstable equilibrium ; the force of the flow is never perfectly 

 vertical to the surface of the water, but always tends in one 

 direction or another and drives the mass away from the point of 

 contact with the stream. 



In order to produce spiral forms precisely similar to those 

 described by Kappers and exhibited by Mr. Burne, I took a 

 round jar full of water and stirred the latter so that it revolved, 

 and then poured the wax on to the surface near the centre. As 

 the mass is rather flat, the wax being much lighter than water, 

 the form produced resembles, as Kappers mentions, a spiral 

 operculum like that of Turbo, and not an ordinary Gastropod 

 shell. "When the water is stirred in a right-handed direction, 

 the spiral is right-handed, and when the water is stirred in the 

 opposite way the spiral of the paraffin plate is left-handed. This 

 alone is sufficient to prove that the resemblance to a shell has 

 nothing to do with crystallization, since the direction of the 

 spiral merely depends on the direction in which the water is 

 revolving. It is to be noted that as one looks down on the 

 paraffin plate, the direction of its spiral is opposite to that of 

 the water, for when the water is moving in a right-handed 

 revolution the movement of the water carries the wax that first 

 falls to the right, and the additions are made to the left; but the 

 form of the spiral shows on the lower surface of the plate of wax, 

 and, of course, when the plate is reversed after it has solidified 

 the spiral appears right-handed. In some cases I obtained 

 spiral forms which closely resembled the internal surface of the 

 operculum of Turbo. The outer surface of this structure is 

 smooth and flat, though it shows the spiral direction of growth, 

 but the inner surface bears a prominent spiral ridge. By 

 pouring the wax at the outer edge of the revolving mass on 

 the water, I obtained plates with a similar prominent spiral 

 ridge. 



The concentric lines on the convex surface of the mass are, as 

 in the molluscan shell, lines of growth or accretion, but their 

 formation requires explanation, since a continuous flow over a 

 smooth surface like that of water, while it would account for the 

 shape, would not be expected to show any lines on the surface in 

 contact with the water. The lines are due probably to the 

 combined effect of surface tension and the slight contraction of 

 the edges of the solidified layer in contact with the water. The 

 molten wax is unable to flow immediately over the edge of the 

 solidified layer, but is heaped up above it until its pressure is 

 too great for the surface tension, when it flows over and comes- 

 into contact with the water, and then the process is repeated. The 

 vibration of the water caused by the fall of the wax may have 



