D. J. SCOURFIELD ON HYDRA AND SURFACE-FILM OF WATER. 139 



But, however sticky a substance may be, it is impossible to 

 imagine that, merely because of its adhesiveness, it can suspend a 

 weight against gravity from the surface-film. Such a supposition 

 would push the analogy of the surface-film to an elastic membrane 

 to the point of absurdity. There is only one way, I believe, by 

 which a weight can be supported by the surface-film, and that is 

 by the formation of capillary depressions. 



This statement probably needs a little amplification to make 

 it intelligible to those who are not familiar with the peculiar 

 properties of the surface-film of water. Without going into all 

 the details of the subject, it may be stated that there are two 

 "Essential facts to be remembered in this connection. The first is 

 chat the surface of water is bent down below the normal horizontal 

 level into little hollows, usually called capillary depressions, when- 

 ever water-repellent substances {e.g., paraftin-wax, fat, etc.) are 

 brouo-ht into contact with the surface, whether from above or 

 below. Secondly, the surface-film bounding the capillary de- 

 pression exerts an upward pull upon the object against w^hich it 

 is formed. A simple experiment, illustrated in Fig. 4, will make 

 this clear. Take a portion of a lucifer match, and weight it at 

 one end by driving in a pin. The weight must now be adjusted, 

 by nipping off portions of the pin or cutting off pieces of the wood, 

 as the case may be, until the match and pin together are slightly 

 heavier than water. If now a small quantity of vaseline be 

 placed upon the upper end of the wood, it will be found that when 

 the match is brought up to the surface a capillary depression quite 

 evident to the naked eye is formed round its top, owing to the 

 water-repellent nature of the vaseline, and that the match and 

 pin, although together heavier than water, ^vill remain suspended 

 as shown in the figure. 



Exactly the same phenomena occur, I believe, when a Hydra 

 is brought up from below to the surface, although they are not 

 so easy to demonstrate. There is no doubt, however, that very 

 minute capillary depressions are formed when a Hydra is attached 

 to the surface-film, because if the surface of the water be 

 examined with a pocket lens at a very oblique angle — that is, 

 in such a way that the light from a lamp or window is largely 

 reflected to the eye, the surface is seen to be broken exactly 

 at the spot where the basal disc of the Hydra is situated. The 

 reason why these depressions cannot be directly demonstrated 



