XII] OF THE FORAMINIFERA 599 



first, has arisen as a drop of protoplasm which exuded through the 

 pores of the first chamber, accumulated on its surface, and spread 

 over the latter till it came to rest in a position of equihbrium. 

 We may take it that this position of equilibrium is determined, 

 at least in the first instance, by the "law of the constant angle," 

 which holds, or tends to hold, in all cases where the free surface 

 of a given liquid is in contact with a given solid, in presence of 

 another liquid or a gas. The corresponding equations are pre- 

 cisely the same as those which we have used in discussing the 

 form of a drop (on p. 294) ; though some slight modification must 

 be made in our definitions, inasmuch as the consideration of 

 snrisice-tension is no longer appropriate at the solid surfaces, and 

 the concept of suvisice- energy must take its place. Be that as it 

 may, it is enough for us to observe that, in such a case as ours, 

 when a given fluid (namely protoplasm) is in surface contact with 

 a solid (viz. a calcareous shell), in presence of another fluid (sea- 

 water), then the angle of contact, or angle by which the common 

 surface (or interface) of the two liquids abuts against the sohd wall, 

 tends to be constant: and that being so, the drop will have a 

 certain definite form, depending {inter alia) on the form of the 

 surface with which it is in contact. After a period of rest, during 

 which the surface of our second drop becomes rigid by calcification, 

 a new period of growth will recur and a new drop of protoplasm 

 be accumulated. Circumstances remaining the same, this new 

 drop will meet the solid surface of the shell at the same angle as 

 did the former one; and, the other forces at work on the system 

 remaining the same, the form of the whole drop, or chamber, will 

 be the same as before. 



According to Rhumbler, this "law of the constant angle" is 

 the fundamental principle in the mechanical conformation of the 

 foraminiferal shell, and provides for the symmetry of form as 

 well as of position in each succeeding drop of protoplasm : which 

 form and position, once acquired, become rigid and fixed with the 

 onset of calcification. But Rhumbler's explanation brings with 

 it its own difficulties. It is by no means easy of verification, for 

 on the very comphcated curved surfaces of the shell it seems to 

 me extraordinarily difficult to measure, or even to recognise, the 

 actual angle of contact: of which angle of contact, by the way. 



