425 
1878 .] Scientific Notes . 
and zinc pair as the resonator. Mr. Lancaster’s Pile Microphone, as he calls 
it, consists of a thin wooden box, 6 ins. x 4 ins. x 1 in., upon the top of 
which is screwed a plate of zinc, 4 ins. x 2 ins., and about one-tenth of an 
inch thick, somewhat smaller than the top of the box. On this is placed a 
layer of blotting-paper soaked in dilute sulphuric acid, and on this again a 
gas carbon-plate, l an inch thick ; a second, but smaller and thinner, plate of 
carbon is suspended over the first, at about 2 inches distance, by being screwed 
to an upright which passes through the plates of the pile into the box below. 
Against the suspended plate rests a thin upright rod or plate of carbon, the 
lower end of which is pointed, and rests in a depression scooped out of the 
lower plate and filled with mercury. Mr. Edison, the inventor of the Phono- 
graph, lays claim to the priority of discovery, on account of his having 
discovered that the compression of a piece of black-lead interposed in a 
circuit proportionately altered its ele&rical resistance, but any discussion as 
to the merits of the rival discoverers would at present be premature and out 
of place. 
At the March meeting of the Physical Society a new battery of extraordi- 
nary capabilities was exhibited and described by Mr. W. H. Preece. It was 
specially devised by Dr. Byrne to be used by medical men in certain cau- 
terising operations. It consists of a simple cell of platinum and zinc, excited 
by a mixture of potassium bichromate, sulphuric acid, and water. The 
negative plate Is compound, being backed with lead to which is soldered a 
sheet of copper, which is again covered with lead. The lead back is covered 
with a non-condudting varnish, the platinum surface being of course left free. 
While in use a stream of air is forced through the liquid by a small air-pump, 
stirring it up a&ively, the heating power being thereby largely increased. The 
a&ion of the air is pioved to be entirely mechanical, and seems to work by 
exposing the zinc plate to successive layers of fresh acid. With an 18-inch 
indudtoiium sparks of over 17 inches could be obtained from a large 10-ceil 
battery, but immediately the current of air was stopped the spark fell to 
8 inches. With a battery of four cells, measuring 4 inches by 2 inches, a 
length of 6 inches of platinum wire, No. 18 (0 05 in.), was heated to bright 
redness. With the larger 10-celled battery no less than 30 inches of No. 14 
were heated. 
At another meeting of the same Society Prof. S. P. Thompson exhibited 
and described a cheap and efficient form of optical bench. Two straight oak 
bars, about 2 metres in length, and clamped together as in a lathe-bed, and a 
number of slides carrying various appliances slide easily without shake, and 
can be fixed in any position by wedges. The several frames carrying the 
diffra&ion grating or edges, the eye-piece (with an engraved glass micrometer), 
&c., are so made, in wood, as to be capable of adjustment in any plane, and 
the instrument can also be employed for making photometric and other 
similar measurements. The mean of two determinations for the wave-length 
of certain red light gave 0*000629 as compared with Fresnel’s figure, 0*000640, 
while the total cost did not exceed £5. 
The following method of precipitating the gold contained in old toning- 
baths is proposed by F. Haugh. The baths when no longer fit for use are 
filtered into a white glass flask, rendered alkaline with a little bicarbonate of 
soda, and a concentrated alcoholic solution of magenta is added drop by drop 
until the liquid has taken the deep red hue of syrup of raspberries. The flask 
is then exposed for six to eight hours to the light of a bright window. At the 
end of this time the gold is found to be deposited as a violet powder, whilst 
the supernatant liquid has become colourless. It is carefully decanted, so as 
to preserve merely the deposit. When a sufficient quantity of protoxide of 
gold has thus been collected, it is carefully washed upon a filter, dried, and 
the filter is burnt. The dry residue and the ash of the filter are then dissolved 
at a gentle heat in an excess of aqua regia , and the solution — diluted with 
distdled water — is separated from the insoluble substance by filtration. 
For the restoration of writing effaced by time M. E. von Bibra proposes to 
