114 On Hollow Pyramidal Ice Crystals. [Feb. 1. 



pyramids, analogous in shape to the well-known cubic crystals of rock 

 salt. The hollow ice pyramids were, roughly speaking, built in the 

 proportion of base 1 to height 2, and ranged up to about 1 in. 

 <liameter of base. They were attached to the wall by their apices, 

 turning their hollow bases towards the interior of the cave. They 

 were only found on those parts where stalactites did not occur. The 

 temperature was +0'5 C. ( + 33 F.), and, as the cave forms a cul-de- 

 sac, the air is perfectly calm. The crystals, having thus evidently 

 been formed from the moisture of the atmosphere, had to be con- 

 sidered as a kind of hoar frost. 



II. Hoar Frost. During Christmas week, 1892, similar forms of ice 

 crystals, though on a smaller scale, were found in an unusually fine 

 hoar frost. These forms comprised simple aiid compound hollow 

 hexagonal pyramids, which were sketched at the time. 



III. Artificial Hoar Frost. Experiments had been planned already 

 before Christmas, 1892, for the artificial production of hoar frost. 

 Before so doing, however, it was thought advisable to search for any 

 possible traces of artificial hoar frost in the refrigerating chambers 

 used for the frozen-meat trade in Liverpool. This visit rendered 

 experiments unnecessary, as it yielded a rich harvest of simple and 

 compound forms of similar hollow pyramidal ice crystals. The 

 chambers having been cooled down to 13 C. ( + 9 F.), it was 

 possible to examine the forms minutely without danger of melting, 

 and photographs and microphotographs were taken by magnesium 

 light. 



Similar forms of artificial hoar frost were found in the refrigerating 

 ss. " Hellopes," in Liverpool, and also in the cooling cellars of 

 breweries in Berlin. 



[IV. During a severe frost in January, 1894, we found in various 

 parts of Cheshire the same hollow pyramidal ice crystals occurring 

 on the nnder surfaces of ice crusts covering hollow ruts in clayey 

 soil, and small pools where an air space divided the ice from the 

 water. No ice crystals were found on the sides and bottom of the 

 ruts. There was no trace of hoar frost on adjacent objects. Micro- 

 photographs were obtained. January 31.] 



Y. Comparison with other Skeleton Crystals (e.g., Bock Salt). 

 Like the " hopper-crystals " of rock salt, the hollow, hexagonal 

 pyramids of ice have to be considered as skeleton crystals. A study 

 of the conditions under which these are formed leads us to dis- 

 tinguish three different types of skeleton crystals : 



o. Those due to overgrowth, e.g., KC1. 



6. Those due to growth at the upper edge in swimming crystals, 



e.g., NaCl. 

 c. Those due to starvation growth, e.g., hoar frost. 



