SCIENTIFIC SUMMARY. 
433 
ceases and scarcely any electricity is transmitted. An electric bell inserted 
in the circuit in the first case rang violently, and in the second case did not 
move. Eeplacing the bell by an iron wire, in the first case it was melted, 
and not even heated in the second. A galvanometer gave in one case a de- 
flection of 22° and in the other of only 2°. The effect is produced instantly ; 
it is constant and durable whatever the number of inversions of the current. 
If other metals are used instead of platinum, they are deposited on the 
aluminum and interfere with the experiment. This stoppage of the current 
is not produced by a plate of gold, silver, platinum, copper, zinc, magnesium, 
tin, lead, &c., replacing the aluminum. A partial effect is produced with 
iron, but the surface is soon altered, with the disengagement of a bad odour. 
As to the aluminum, its surface appears to be preserved by a slight layer of 
alumina, which is formed immediately, and remains, in spite of the inver- 
sion of the current. Many practical applications of this property suggest 
themselves. Two messages may be sent over a telegraph line at the same 
time in opposite directions by using two voltameters with the aluminum on 
opposite sides. All trouble from variable resistance is thus avoided. 
Effects of Heat on the Structure of Steel Rods and Wires. — A paper was 
read before the British Association by Professor W. F. Barrett, in which he 
said he found that if steel of any thickness be heated by any means, at a 
certain temperature the wire ceases to expand, although the heat be con- 
tinuously poured in. During this period also the wire does not increase in 
temperature. The length of time during which this abnormal condition 
lasts varies with the thickness of the wire and the rapidity with which it 
can be heated through. It ceases to expand, and no further change takes 
place till the heat is cut off. When this is done the wire begins to cool 
down regularly till it has reached the critical point at which the change 
took place on heating. Here a second and reverse change occurs. At the 
moment that the expansion occurs, an actual increase in temperature takes 
place sufficiently large to cause the wire to glow again with a red-hot heat. 
It is curious that this after-glow had not been noticed long ago, for it is a 
very conspicuous object in steel wires that have been raised to a white heat 
and allowed to cool. 
Supporting Crucibles in Gas Furnaces. — This important subject was lately 
brought before the Chemical Society by Mr. C. Griffin. In the author’s 
gas-furnace, a description of which was communicated to the Society in 
1870, the perforated plumbago cylinder, and the trivet-grate on which the 
crucible is supported, are liable to break when white-hot, occasionally giving 
much trouble ; moreover, the latter has the disadvantage of interfering with 
the direct action of the flame on the crucible. This, however, is entirely 
obviated by the new burner, in which a space is left round the central jet 
which has fitted over it an atmopyre similar to those used in Hofmann’s com- 
bustion-furnace. The bottom of the crucible rests on this, and the plumbago 
cylinder is thus relieved of all pressure. These new burners are very econo- 
mical and of great power, a small one, burning 20 feet of gas per hour, being 
capable of melting half-a-pound of cast-iron in thirty-five minutes ; or of 
heating a muffle, 5 inches long and 3 wide, to a temperature sufficiently 
high for assaying. Several varieties of the furnace were exhibited, one of 
which was in action. 
F F 
YOL. XIY.— NO. LVII. 
