KTOVEMBER 9, 1906.] 



SCIENCE. 



598 



the curve of its plastic yield-point in the direc- 

 tion of increasing temperature or pressure. 



As to the internal constitutive differences 

 which distinguish the fluid from the liquid 

 state much remains to be learned, but it is 

 regarded as probable that these differences 

 are to a large extent grounded in a difference 

 in molecular complexity; fluid and liquid 

 doubtless bear to one another the same rela- 

 tion as vapor and gas, which usually always 

 differ in molecular weight. Thus, sulphur 

 vapor at 445° C. has the molecular formula 

 Sg; as the temperature of this vapor is raised, 

 the complex molecule Sg undergoes a disin- 

 tegration and finally becomes S^, which is not 

 broken up until the temperature has become 

 very high. At a point which we may call the 

 simple gas-point the complex molecules of the 

 vapor phase give place to the simple molecules 

 of the gas phase. Both boiling-point and 

 simple gas-point vary with the temperature 

 and pressure, and hence both may be repre- 

 sented graphically by curves. The vapor 

 phase or condition will lie between these two 

 curves, and such a phase is common to prac- 

 tically all substances that can be boiled. 



Now when a solid is heated under atmos- 

 pheric pressure it first softens and then melts ; 

 and this intermediate stage of softening, when 

 the substance is neither solid nor liquid, is 

 properly the fluid phase or state, comparable 

 to the vapor phase which is intermediate be- 

 tween liquid and gas. It can hardly be 

 doubted that in the solid phase the molecular 

 weight is the largest possible. When the 

 yield-point is attained, these large molecules 

 just begin to break up into smaller units and 

 this process is continued until the melting 

 point is reached and the substance becomes a 

 liquid; in this condition the molecular com- 

 plexes are very stable, and, like the simple 

 gas molecules, generally require large addi- 

 tions of energy to undergo further change. 

 Oswm W. WiLLCox. 



Sandy Hook Proving Gbounb, 

 Fort Hancock, N. J. 



WHY DO HERRING GULLS KILL THEIR YOUNG? 



Early last July I was making for the Public 

 Museum of Milwaukee a photographically re- 



corded study of the actions of a colony of 

 herring gulls covering Gravel Island, a little 

 islet at the upper end of the Door County 

 peninsula of Wisconsin. At the time of my 

 visit the young of the year were mostly about 

 half grown and usually kept by themselves in 

 a flock of several hundred. There were al- 

 ways a few young scattered among the adults 

 on the island, but the great majority were at 

 all times to be found in the flock, which 

 seemed to have taken to itself a definite 

 habitat at one part of the islet. 



The island is, I am told, visited with 

 some degree of frequency by pleasure-seekers, 

 though as it is fairly remote from any resort 

 I do not fancy that these visits are very nu- 

 merous in any one nesting season. 



When first visiting the island I rowed out 

 to it with two companions, pitched my tent 

 in the center of the island and disappeared 

 within, after which my companions returned 

 to the boat and rowed away. I have reason 

 to believe that my remaining among the gulls 

 was not suspected by them. From various 

 observations I am convinced that the indi- 

 vidualities of the young were so hopelessly lost 

 in the flock that they neither knew nor were 

 known by their parents. 



Almost from the beginning of my obser- 

 vations and continuing intermittently all 

 through them I was witness of numerous at- 

 tacks committed by adults upon the young, in 

 which the latter were so severely wounded that 

 death did or in a short time would ensue. At 

 the time of my arrival this year and also 

 when I visited the same colony a year ago 

 the island was strewn with the bodies of some 

 dozens of these half-grown gulls bearing evi- 

 dence of having been similarly done to death. 



Usually when my attention was attracted 

 to the enactment of one of these tragedies the 

 victim was a bird to which I had paid no 

 previous attention; but there were several 

 instances where the young had been under 

 fairly close observation for some hours pre- 

 vious. In no instance could I see that the 

 young was weak, sickly or in any way ab- 

 normal, nor that it had given offense for which 

 it was being punished. Sometimes when the 

 flock of young was on the water near the 



