562 



Dr. R. Norris on the passage of [June 15, 



that, in fact, the puckered or surplus material should be taken up. "We 

 may say, then, that any film which can adapt itself to the surface of a 

 spherical body must possess the twofold quality of facile contraction and 

 expansion, these qualities being controlled in their operation by progres- 

 sive cohesive attraction. Such a film must be a simple colloidal cohesion- 

 membrane in possession of the property of " flow." 



Further, if we apply to a sphere a film known to possess facile proper- 

 ties of expansion and contraction under the influence of slight forces, such 

 as progressive cohesive attraction, the first thing seen to occur is cohesion 

 of the film to the sphere at the point of contact ; and from this point as a 

 centre of operation the film proceeds to apply itself gradually in all direc- 

 tions, so that the sphere becomes coated or covered evenly by it : this 

 process goes on till such time as the attraction becomes balanced or fully 

 antagonized by the elasticity of the film, that is to say, the attraction is only 

 powerful enough to stretch the film to a certain extent ; so that if the rigid 

 object be fixed, as is the case with the glass bulb when held immovably, 

 we get a flattened form of the film. A sufficient degree of attachment of 

 the film to the bulb has taken place to stretch the former backwards out 

 of its normal plane. If, now, we push the bulb further forwards, the film 

 still continues to apply itself to its surface, and having reached the equato- 

 rial line of the sphere, descends on the opposite hemisphere till the bulb 

 is completely coated. But it will be said the bulb does not then really 

 produce an infraction of the film, but merely attracts it down to its surface, 

 and in so doing stretches it, so that it is in reality a new conformation of 

 the film and not a breach of its continuity. That this is true to a certain 

 extent there is no doubt, but it is not all the truth ; for we may wipe the 

 bulb dry after it has passed through the film without interfering with the 

 continuity of the latter. All that appears to be necessary for these effects 

 to display themselves is, that there should be mutual cohesion between the 

 film and the body passing through it ; for if we press against one of these 

 delicate films with a substance which has no cohesion for it, e. g. a current 

 of air or a dry soap-sphere, it simply distends the film, neither bursting it 

 nor giving rise to an aperture in it ; while in the case of a body to which the 

 film can cohere, it would appear to be easier for the latter to allow the 

 passage of the cohering body than to suffer distension by it, and this be- 

 cause it has under these conditions as great an attraction for the particles 

 of the body as for its own particles. When the cohering body has become 

 perfectly applied to the film, the latter, by the cohesiveness of its own 

 particles, contracts to the greatest degree possible consistent with still main- 

 taining its attachment to the cohering body ; and this in spherical-shaped 

 bodies leads to a condition of things in which half the body is within and 

 half without the film or wall*; therefore the rest of the process must be 



* An excellent illustration of this principle is afforded when a light india-rubber ball 

 or balloon is suspended from a fixed point, its surface having previously been wetted 

 with a solution of soap. When a soap-film formed upon the ring, as in the previous 



