264 Chronicles of Science. [ April, 
temperature a tension of dissociation which is measured by the 
elastic force of the aqueous vapour which it emits at this tempera- 
ture. Applying this to the explanation of the phenomena of 
hydration and efflorescence, he states further that a salt becomes 
hydrated when the tension of the aqueous vapour contained in the 
atmosphere is greater than that which the salt emits at the same 
temperature. LEfflorescence results when the tension of the water- 
vapour of the salt is greater than that of the aqueous vapour existing 
in the atmosphere. Hydrous salts which do not effloresce owe, 
then, this property to the inferior tension of aqueous vapour emitted 
by them at common temperatures to that ordinarily possessed by 
the aqueous vapour in the atmosphere. These identical salts effloresce 
when placed in an atmosphere where the elastic force of the aqueous 
vapour contained in the air is below that which they emit. 
Professor Knoblauch has made an investigation on the inter- 
ference-colours of radiant heat. Some of his results are embodied 
in the following :—When two groups of rays meet under certain 
conditions, radiant heat differs in its properties after the meeting ; 
for instance, as regards its property of traversing diathermanous 
bodies, it manifests an interference-colour. If this is produced in 
doubly refracting crystals, under the influence of a polarizing agent, 
placed for instance between two Nicol’s prisms, and the plate of 
crystal fixed while one prism is rotated, the colour passes through 
white to the complementary. On rotating the plate of crystal in 
its plane, when the principal sections of the Nicol’s prisms are 
parallel, only one thermal colour occurs; when they are at right 
angles, the complementary colour; while wheu they form an angle 
of 45°, both thermal colours appear. 
Experiments have been made in Germany which tend to show 
that molten lead dropped upon liquid iron remains floating on the 
surface of the latter. Since the specific gravity of lead is more 
than one-half greater than that of cast-iron, the fact seemed anoma- 
lous. Professor Karmarsch, of Hanover, has explained the matter 
very satisfactorily. Some samples of these drops of lead lying 
embedded in the surface of a cast-iron block were sent to the Pro- 
fessor by an ironmaster. Professor Karmarsch found, upon close 
examination, that these drops of lead were not solid globules, but 
hollow, formed apparently as bubbles. According to this explana- 
tion, the lead is kept resting on the surface of the iron by its own 
vapour. In large quantities the result is known to be different, 
lead being occasionally tapped from the bottom of blast furnaces 
employed in smelting certain classes of ores. 
The value of petroleum as fuel for steamship boilers has been 
for a considerable time under investigation by the United States 
