286 
GILBERT. 
Tycho, surrounded by a relatively dark band inside the 
bright nimbus? 
As no dark rays emanate from the dark craters, it may be 
inferred that the white substance is peculiar in its tendency 
to fly about at the instant of collision. It is probably a 
readily fusible solid. If the ring of moonlets and the earth 
were once the outer and inner parts of the same whirling 
mass, the moonlets should consist of substances somewhat 
abundant in the earth’s crust. Inquiry may therefore be 
addressed to familiar fusible substances of pale color. Fusi¬ 
bility in this case is measured by a factor involving the 
initial temperature, the melting temperature, the specific 
heat, and the latent heat of fusion; and this must be com¬ 
pared with the amount of energy expended in collision. In 
the short table I have constructed, the energy necessary to 
melt a unit mass of a substance is expressed as a fraction of 
the energy of motion destroyed by the collision of a unit 
mass. The ratios were computed on the postulates, first, of 
an initial temperature of —273° C.; second, of an initial 
temperature of —100° C. Ratios are added for the rock 
diabase, as a possible representative of the less fusible moon- 
lets. 
Relative energy 
Relative energy 
Substance. 
for fusion from 
for fusion from 
—273°. 
—100°. 
Tin.... 
.06 
.05 
Phosphorus.... .. 
.11 
.05 
Sulphur..... 
.07 
Silver.... 
.15 
.13 
Nickel*... 
...30-f 
.27+ 
Ice..... 
..33 
.19 
Calcium sulphate*.. 
.43+ 
.38+ 
Sodium chloride... 
. ....46 
.40 > 
Diabase... 
...69 
.61 
Attention is naturally directed to ice by reason of its 
abundance on the earth and its whiteness. If it exists on 
the moon as a solid or liquid, it must also exist as a gas, for 
it would evaporate until the resulting atmosphere had a 
* Latent heat of fusion not known, and ratio therefore too small. 
