60 THE YOUNG 
the telephone disk, a straight or "bar" magnet 
being usually the kind used in the instrument. 
The tuning fork is mounted on a sounding board. 
Similar apparatus is provided at both ends of the 
line. To sound the alarm, in order to give a call of 
the telophone, it is only necessary to draw a fiddle- 
bow across one of the tuning forks; the other, at 
the further end of the line, responds with a note 
loud enough to attract attention throughout a 
large room. 
Inquiries. 
10. Can any of your readers tell me how to make 
something to attach to my gas burner, so that I 
can light my gas with a battery. G. F. P. 
11. As I was reading the last number of your 
valuable magazine, I came across an article on 
camphor barometers; and being desirous of making 
one, and not knowing the exact composition or 
proportions, I would respectfully ask you to inform 
me in your next number. G. Y. 
12. If lead should be used for filling teeth, would 
it poison the gums ? E. S. 
13. Will you please inform me what acid will eat 
the vegetable matter from a green leaf, and leave 
the animal substance, making a skeleton leaf. 
J. F. M. 
14. If you could tell me how to make a friction 
polish for turned work, to be applied in the lathe, 
such as we see on the wood on sale in the shops, I 
would be greatly obliged. The beeswax, alcohol, 
sandarac and turpentine formula, given in all the 
books, will not work with me at all. H. H. 
15. Can any of the readers of the Young Sci- 
entist tell me how to mount objects for use with 
the Craig microscope. M. T. F. 
16. What are Italian images and dolls made of? 
Also, how can artificial maple sugar be manufac- 
tured? + 
Answers. 
17. I do not know that my experience will be of 
any use to " Traddles," but it is rather interesting 
both as to how to keep tadpoles, and how not to 
keep them. Several years ago I had charge of an 
aquarium containing young tadpoles and several of 
our common fresh water fish — I have forgotten 
what kinds. The fish and the tadpoles agreed very 
well together. The tadpoles flourished, with no 
special attention, increased in size, and after due 
time developed their legs, and would no doubt 
have become good honest frogs— but then the fish 
eat them up. This was a phase of evolution that I 
had not before heard of. Cakl Reddots. 
18. In answer to "Amateur's" inquiry how to 
preserve lizards, frogs, etc., I beg to inform him 
that by using the "preserving fluid for scientific 
purposes," he can keep his specimens in their per- 
fectly natural state, without any previous prepara- 
tions. See advertisement. 
Hermann Hurtzig, P. 0. Box 4920, New York. 
19. Some tadpoles seem to live largely or wholly 
on vegetable matter, chiefly the green matter that 
grows on the sides of aquaria exposed to the light. 
I have seen them mow this green growth off in cir- 
SCIENTIST. 
cular sweeps, as if done with a miniature scythe. 
The tadpoles of some species of frogs feed on dead 
animal matter. Try them with a few pieces of 
meat. It is a curious fact that tadpoles kept in the 
dark do not develop into frogs. I have kept them 
in a dark cellar for two years, and they were still 
tadpoles; brought to the light they became frogs. 
Ran A. 
20. "Amateur" (letter 5) will find very complete 
directions for preparing and preserving material for 
dissection and microscopical examination in Ruth- 
erford's "Outlines of Practical Histology," pages 
3-7. For mere preservation, Rutherford recom- 
mends proof spirit — our ordinary alcohol will do. 
Where the tissues are to be hardened, they are first 
soaked in a 4 per cent solution of chromic acid— 
that is, one grain of chromic acid dissolved in 400 
grains of water. Some portions of the animal are 
soaked in solution of common salt. The chapter 
is well worth the attention of "Amateur." 
Rana. 
21. " H. W.," (letter 6) does not state the special 
object for which he wishes the cement. If he 
wishes to fix a flat piece of brass on glass or vice 
versa, good marine glue is the best thing he can 
get. If he wishes to fix a brass cap on a glass 
flask or tube, good red sealing-wax will do very 
well. The proper cement for the latter purpose is 
the so-called electrical cement. P. J. 
EXCHANGES. 
In this column yearly subscribers who may wish 
to exchcunge tools, apparatus, books, or the pro- 
ducts of their skill, can state what they have to 
offer and what they want, without charge. Buying 
and selling must, of course, be carried on in the 
advertising columns. 
Wanted, a xylophone in good condition; a Cen- 
tennial jig saw, bought in December, in exchange. 
C. B. Culver, 131 East Seventeenth St., New York. 
Wonderful magic tricks, to exchange for books, 
etc. Leonard Alexander, Linneus, Me. 
Back numbers of the " Youth's Companion," for 
the years 1876 and 1877, cost $1.75 a year; lathe, for 
Centennial bracket saw; or "The Speaker's Gar- 
land," Vol. 2 or 3. F. R. Miller, 750 East Fourth 
street. South Boston, Mass. 
Scroll saw wanted in exchange for handsome 
portfolio of six water color sketches. Address 
F. S., care Box 4875, New York. 
Two Selden's patent printing telegraph instru- 
ments; will exchange for microscope or books. 
Address W. Chamberlain, 446 Broome street. New 
York. 
Specimens of the marbles, granites and minerals 
of Vermont, in exchange for Western minerals, or 
good fossils; minerals and fossil woods from the far 
West specially desired. Dr. H. A. Cutting, State 
Geologist, Lunenburgh, Essex County, Vt. 
Thorough and practical instruction in shorthand 
will be given in exchange for a microscope, with or 
without accessories, worth from $5 to $10. Address 
T. P. Wendover. 68 Christopher street. New York. 
Wanted, a small turning lathe, about 1^ inch 
swing, and 12-inch bed; must be well made; books 
and apparatus in exchange. R. M., care of this 
journal. 
Wanted, a copy of Holtzapfel's "Mechanical 
Manipulation." State what is wanted in exchange. 
E. W., Box 4875, New York. 
