232 
that perhaps here les the answer to the aston- 
ishing performance of nature. 
Dr. Mason—Yes, indeed, that is right. 
Dr. U. Nakaya—My diagram was obtained 
in the case of mixing or convection and in 
your case the crystal is produced by diffusion 
of vapor. The experimental procedures are dif- 
ferent, still both diagrams are quite similar, al- 
though a little different im some portions. The 
most characteristic type of snow crystal, the 
dendritic crystal, appears in the same tempera- 
ture range in both diagrams. One thing to be 
mentioned is that the meaning of supersatura- 
tion in our case is different from the ordinary 
definition. In the case of mixing, usually minute 
fog particles are abundantly produced in the 
atmosphere where the crystals are made. We 
measured the total water content (minute drops 
plus vapor) by the gravimetric method, and 
calculated the degree of supersaturation. Our 
supersaturation, therefore, means the sum of 
water vapor and minute drops. It is very in- 
teresting that these droplets behave just like 
water vapor when they are very small, say, the 
order of one micron in diameter. Watching the 
process of growth of a snow crystal through a 
microscope, it is observed that the minute drops 
DISCUSSION 
spread over the surface of the snow crystal 
without leaving any trace of the drop shape. 
Ii the drops are larger, say, several microns in 
diameter, they freeze in a drop shape on the 
surface of snow, thus giving the rimed snow 
crystal. 
Dr. Mason—I think the mechanism respon- 
sible for the habit changes of ice crystals must 
be a surface phenomenon, because otherwise 
one might expect some difference between heavy 
and ordinary water. This one does not get; 
in fact, I can produce all shapes by adding in- 
creasing but still very small quantities of al- 
cohol. It must be a question of the properties 
on the surface, and changing the surface prop- 
erties. It can not be a question of the structure 
of the ice itself. This is one thing one has to 
worry about; if there is a minute amount of 
impurity, the experiment can come to grief. 
Dr. Nakaya—We must be very careful in 
this point. Sometimes a very small amount of 
impurity changes the shape of crystal com- 
pletely. We have the experience that an inde- 
tectable amount of silicone vapor changed the 
erystal shape completely. 
Dr. Mason—This I would expect. 
