724 



and others, using greater precaution, and have invariably obtained 

 similar symptoms of urtication. In only one instance has a minute 

 ulceration been the consequence. 



It is very evident therefore that the Actiniae act by means of an 

 acrid irritant poison, similar in some respects to that of the wasp, or 

 of snakes, which quickly spreads through the system of the Annelida, 

 producing the above-mentioned results. 



It remained to determine whether the poisoned weapons existing 

 in such numbers over the surface of the Actiniae were left in the part 

 attacked. For this purpose I stretched a thin India-rubber mem- 

 brane over a glass tube. After its seizure by the Actinia, I found 

 that under the microscope it was studded in many points with the 

 poison darts inserted slightly in the membrane, without their having 

 penetrated through. In this respect my observations differ from 

 those of Mr. Gosse, who considers that a fragment of cuticle from 

 the hand was perforated by these darts. 



I remain, &c., 



AUGUSTUS WALLER. 



II. " On the Double Tangent of a Plane Curve." By ARTHUR 

 CAYLEY, Esq., F.R.S. Received March 17, 1859. 



(Abstract.) 



The author notices that the problem of finding the number of 

 double tangents was first solved by Pliicker in 1834 from geometrical 

 considerations, and he gives a sketch of the subsequent history of 

 the problem. The complete analytical determination of the double 

 tangents was only obtained very recently by Mr. Salmon, and is given 

 in a note by him in the Philosophical Magazine, October 1858 : it is 

 there shown that the (n 2) points in which the tangent at any point 

 of a curve of the order n again meets the curve, are given as the 

 points of intersection of the tangent with a certain curve of the order 

 (n 2) ; if this curve be touched by the tangent, then the point of 

 contact will be also a point of contact of the tangent and the curve 

 of the order n, or the tangent will be a double tangent. The pre- 

 sent memoir relates chiefly to the establishment of an identical 

 equation, which puts in evidence the property of the curve of the 



