199 



pressod-oiit plant sa|), even when, afler repeated l)oiling 

 and lilt('riiii>', the licjiiid was clear and transparent, he did 

 ()l)1ain subeooling at will, when he had previously fi-ozen 

 and thawed that clear liquid several times and let the air 

 bubbles enclosed in the ice escape during thawing. To 

 prevent the dissolving of air bubbles anew, he maintained 

 a layer of oil at the surface of the material. The difficulty 

 of subeooling tissues which contain tracheae is attributed 

 by him to the abundance of air in them. He explains in 

 the same manner the fact that the intercellulars freeze 

 before the cell contents. Concerning the mechanism of 

 action of the air bubbles, Mez suggests that their separa- 

 tion under the action of cold consumes heat and that the 

 resulting local cooling favors the formation of crystal 

 nuclei. 



Some authors think that the action of air in inducing 

 crystallization is indirect and that it should be attributed 

 to dust particles or infinitesimal crystals contained in the 

 air. But against this contention one has the observation 

 of Moran (1925) that chicken eggs show an increase in the 

 tendency to subcool when their shell is coated with vaseline. 



Cases of crystallization of a subcooled liquid in a sealed 

 glass container, at the breaking of the latter, are not un- 

 frequently mentioned in the literature. Some authors 

 attribute crystallization to a disturbance at breaking, 

 others, to the sudden contact with the air. 



Mez (1905) could not readily obtain subeooling in cell- 

 sap wdiich had been heated wdiile it was covered by a layer 

 of oil. He thinks that some emulsification took place and 

 that the oil droplets in the sap play the same role as air 

 bubbles in preventing subeooling. 



The efficacy of colloidal micelles in inducing crystalliza- 

 tion is suggested by Flichtbauer (1904) who expresses the 

 opinion that some dust particles which prevent subeooling 

 are of colloidal nature. 



Mez (1905) considers the difficulty in subeooling turl)id 

 cell-sap as due partly to the action of mucous and gummy 

 substances. He thinks that colloidal particles might give 

 the start to the formation of nuclei. 



