959 
after the opening of the lamp, the deposit showed a network of 
small, radiant ultramicrons. In course of time this network became 
distinctly coarser and after some hours separate particles could be 
discerned, while after a still longer period some of them clearly 
revealed the cubic form of rock-salt crystals. Hence a slow cry- 
stallization or re-crystallization of the rock-salt takes place. 
This change caused by the addition of the moist air made it 
impossible to observe the layer of rock-salt in its initial state. In 
order to do this it should be shut off from the gasphase, both in the 
course of and after its formation, and after the opening of the 
balloon it should remain shut off as well. 
Closure with oil, obtained by opening the balloon under oil, so 
that the latter entered instead of the air, proved unsuccessful. A 
better result was obtained by using Canada Balsam, which had been 
freed from dissolved gases by exposure to high vacuum under gentle 
heating, by which process the balsam got thickened. During this 
operation it had been present in a side-tube, which had been fused 
to the balloon. After the formation of the NaCl-film the Canada 
Balsam is made into a thin fluid by cautious heating and is spread 
out over the layer of salt, so that it was partly covered by the 
balsam. When the lamp was then opened, the layer of salt under- 
neath the Canada-Balsam remained perfectly clear and transparent, 
whereas in the uncovered places it turned white. 
In the ultramicroscope the layer covered with Canada Balsam was 
optically empty. 
This became quite obvious when adjusting at the boundary-line 
Canada Balsam-air. Where the air had been allowed to operate the 
field was strongly lighted by the network of small ultramicrons, 
where the Canada Balsam had protected, the field was dark. The 
two fields were separated by a very distinct line of demarcation. 
So the rock-salt is deposited on the bulb as an optically homo- 
geneous phase, crystals are altogether absent, the molecules are 
absolutely disordered, and we are dealing here with the amorphous, 
vitreous condition, that may be compared to the undercooled fluid. 
This condition is metastable and gradually passes into the crystal- 
lised, stable condition. Various circumstances may start or accelerate 
this conversion : 
1. Access of air. Water vapour especially plays an active part 
here, very dry air has hardly any effect, breathing upon it makes 
the conversion take place very rapidly. 
2. A Rise of Temperature. 
61* 
