336 
POrULAR SCIENCE REVIEW. 
which came within his experience at Delft, in Holland. It appears that, in 
order to obtain fresh water for domestic purposes, a so-called Norton pump- 
tube was being rammed down in the alluvial soil (peat-bog chiefly). 
When at a depth of 17*5 metres below the surface, and very nearly as much 
below sea-level at that locality, the ram-block was suddenly lifted by a mass 
of gas issuing from the opening of the iron tube, immediately followed by 
water, which rose to a height of 14 metres, and continued to do so for 
some fourteen hours j after that lapse of time, the fountain became for a few 
weeks intermittent. The temperature of the water was 13°. The composi- 
tion of the gas, in 100 parts, by bulk, was found, by Dr. A. C. Oudemans, 
jun., to consist of 91*2 of marsh gas and 8*2 of carbonic acid, with traces 
only of air, while carbonic oxide and heavy hydrocarbons were proved to be 
absent. The phenomena alluded to ceased when, after some weeks, the 
iron pump-tube was forced down to a depth of 25 metres. 
Iron as a Filter and Deodoriser. — Attention has been called to the use of 
spongy iron as a deodorising material which, Dr. Voelcker considers of greater 
potency than animal charcoal. Sewage water passed through a filter of this 
substance is completely purified, and this filtered water, after having been 
kept six months protected from the air, was perfectly sweet, and free from 
any fungus growth. The spongy iron is obtained by calcining a finely 
divided iron ore with charcoal. Mr. Spencer, whose name is connected 
with the discovery of the electrotype, has for some time been advocating the 
use of a filter of this description. Its power of rendering water beautifully 
transparent, and apparently free from all organic matter, is its strong 
recommendation. We ourselves have had a considerable experience of the 
Spencer filter, and we consider it an admirable one. 
A Drop of Water on a Hot Hate. — The u Society of Arts Journal ” 
(May 26), quoting from PoggendorfTs 11 Annalen,” gives an account of an 
experiment made by E. Budde, to ascertain whether a Leidenfrost’s drop 
with water could be produced at a less temperature than 100 C. The ex- 
periment was made by letting a drop of water fall upon a hot plate covered 
by a partially exhausted receiver, and it was found that the drop assumed 
the spheroidal condition at a temperature of 85 C., confirming the doctrine 
that the force which supports the drop obeys the laws of the pressure of 
vapours. The star shape assumed by the drop Berger was explained to be a 
phenomenon of vibration. If the drop is large it behaves like other 
vibrating bodies, and divides into aliquot parts, forming nodes or loops. 
Depolarisation of Iron Ships. — Mr. Charles E. Henwood, a member of 
the Institution of Naval Architects, read a paper at the late meeting of the 
Institution (March 30) on the invention of the late Evan Hopkins, C.E., F.G.S., 
by which the sub-permanent magnetism of an iron ship, the principal cause 
of the deviation and consequent depreciation of the compass, is stated to be 
completely and permanently removed. The paper contained a number of 
extracts, and a table showing the deviations of the compass of H.M.S. 
Northumberland before and after depolarisation, which will be given in the 
u Transactions ” of the Institute. 
A Colour Experiment in Optics. — In a late number of the “ Proceedings 
of the Boyal Society ” Sir Charles Wheatstone says that few experiments in 
physical optics are so beautiful and striking as the elegant pictures formed 
