592 



NATURE 



[April 19, 1900 



by the net system, in which case the meshes on either 

 surface of the ice-plate are of equal size. 



As to the explanations which have been offered to 

 account for the formation of the honeycomb structure, no 

 theory seems to have been recently proposed which is 

 not based upon that of Robert Emden, who supposed 



ice. The peculiar form of the network developed, for 

 instance, on the surface of an ice-stalactite, with its 

 radially arranged tiers of "cells," requires further ex- 

 planation. Lohmann ascribes an important part in de- 

 termining the arrangement of the " cells " to the expansion 

 and contraction of the ice under changes of temperature. 

 Since the coefficient of expansion of ice is great, the 

 surface when exposed to variations of temperature will 

 undergo a splittmg process, which will result in the pro- 

 duction of so-called "elementary cells." These will then 

 become crystal units (if not already such) by the process 

 of coalition which Emden supposed to occur, and the 

 ultimate dimensions of the prismatic structure are then 



Structure of exposed surface of wall-ic 



that in all ice masses exposed for some time to a tem- 

 perature neighbouring on the freezing point, the minute 

 first-formed crystals, even when possessing an irregular 

 original arrangement, tend to undergo a complete re- 

 arrangement, and to form crystal units of ever-increasing 

 size, having similarly directed axes. The dimensions to 

 which these new crystal units will attain is determined 

 by the temperature variations. Should the temperature 

 rise and a thaw set in, those portions of the ice where 

 the individual crystals are in contact with one another 

 will be the first to be attacked, and the melting process 

 will produce the appearance of the net system. 



Since the rock in which the caves or drifts are formed 

 always possesses a somewhat higher temperature than the 

 air within the cavity, the coalition of the individual ice i 

 crystals in contact with the cave wall will take place at a i 

 somewhat earlier period than that of the crystals on the 

 exposed surface of the wall-ice, for the inner side will be 

 sooner raised to a temperature closely approaching the 

 freezing point. But in like manner, as the general tem- 

 perature rises, the inner surface of the wall-ice will 

 become exposed to the action of actual thawing earlier 

 than the free outer surface, and the development of the 

 growing crystal units — the groundwork for the "cells" of 

 the honeycomb structure— will receive an earlier check 

 on the inner surface than on the free surface. This ex- 

 planation has been given to account for the fact that the 

 meshes in contact with the rock are smaller than those 

 exposed to the cave air, as shown in the accompanying 

 illustrations reproduced from Dr. Lohmann's paper, for 

 which photographs were obtained from plaster casts of 

 the ice surfaces. 



But Emden's theory, as briefly sketched above, does 

 not seem to explain certain facts observed in connection 

 with the " prismatic " structure shown by some forms of 



NO. 1590, VOL. 61] 



Fig. 2. — Structure of ice surface in contact with rock. 



finally determined by a rise of temperature, forming 

 spaces between the " cells " as above indicated. But 

 while this improved theory seems to offer a fuller ex- 

 planation of the observed facts, final proof of its value 

 as a sound hypothesis is only to be sought in the 

 evidence of further experimental investigation. 



F. L. K. 



NOTES. 



Prof. A. A. Michelson, professor of physics in the Uni- 

 versity of Chicago, has been elected a correspondant of the 

 Paris Academy of Sciences. 



The first meeting of the International Conference for the 

 Protection of Wild Animals in Africa will be held at the Foreign 

 Office on April 24. The British representatives will be the 

 Earl of Hopetoun, G.C.M.G., Sir Clement Hill, K.C.M.G., 

 C.B , head of the African department of the Foreign Office, and 

 Prof. Ray Lankester, F.R.S., director of the Natural History 

 Museum. 



To encourage the study of aerial navigation, a member of the 

 Aero Club of Paris has given the sum of 100,000 francs, to be 

 awarded as a prize to the inventor of an efficient aerial machine. 

 The test to be applied is that the machine shall travel from the 

 grounds of the Club, or from the hills of Longchamps, to the Eiffel 

 Tower, and then to return to the starting point. The length of 

 the whole journey is about eleven kilometres, and it has to be 

 accomplished in half an hour or less. The competition is inter- 

 national, and the offer will remain open for five years from the 

 middle of the present month. The interest upon the sum 

 placed at the disposal of the Club will be awarded annually for 



