THE YOUNG 
•which will be luminous when darkness sets in. 
Who can say, indeed, that a policeman, when 
smeared in this luminous composition, will not 
be sometimes visible on his beat at night? 
The following method of procedure will, it is 
said, give a very satisfactory luminous paint : 
Take a number of oyster shells, cleaned from 
organic matter as thoroughly as possible, and 
burn them in a strong coal lire for about half 
an hour, at the end of which time take them 
out and allow to cool. When quite cold, pound 
them fine, removing, during this operation, any 
particles of gray matter that may show them- 
selves, as these are useless. When finely pow- 
dered, make an intimate mixture of this with 
flowers of sulphur. Introduce the mixture into 
a crucible, luting on a lid to the vessel with 
clay, or other convenient luting material. When 
this is dried, place the crucible in the fire and 
allow it to remain for an hour ; then remove, 
and allow to cool before opening. The mixture 
then should appear pure white. Any gray par- 
ticles that have escaped removal at the first 
preparation, should be removed now. The re- 
sulting powder should be mixed with gum 
water to a thin paint, as two thin applications 
are better than one thick one. This paint will 
remain luminous far into the night, provided it 
is exposed to the light during the day.— Build- 
ing News. 
PolisMng Glass. 
Besides the oxide of zinc (commonly called 
ashes of zinc), is usually used for the cutting 
and polishing of glass. In glasses intended for 
optical purposes, the very finest polish can, of 
course, be only obtained by exceedingly fine 
grained oxide of iron. But since such an arti- 
cle can not always be obtained, and even oxide 
of iron prepared after Vogel's receipt from 
oxidulated protoxide of iron does not always 
answer the purpose, the glasses polished with 
it sometimes looking dim and brown yellow, 
because the finest particles of the preparation 
enter the pores of the glass. Professor Dr. J. I. 
Pohl made experiments with oxide of zinc. The 
result answered, as Dingler's Polytechnical 
Journal states, perfectly the expectations enter- 
tained. The sorts used were: 1. The finest 
white zinc, such as the factory of Petes waid, in 
Silesia, puts into the market. Of these the grey 
zinc was found particularly well adapted to cut 
glass, while the Hamburg white answered for 
common polishing. The finest white zinc was 
found to be excellent for the so-called " high 
polish " of optical glasses. White zinc does not 
only polish quickly, but with the finest sort the 
very highest white brilliancy can be obtained. 
For polishing the very expensive optical 
SCIENTIST. 83 
glasses. Prof. Pohl considers the white zinc 
known in France under the name of blanche de 
niege (i. e., snow white) best adapted. 
JJractual ^intQ. 
To Clean Bronze Statues.— The objection- 
able dark coating which most bronze statues 
soon acquire, with the look of cast iron, may be 
very quickly and completely washed off by means 
of a concentrated solution of carbonate of am- 
monia, applied with brushes. Thereupon a 
layer of patina is formed, which guards the 
statue against fresh formation of the dark coat. 
The work should, of course, only be entrusted to 
skilled and intelligent men. Another method is 
to paint the statue, at intervals of a few weeks, 
repeating with a solution of 20 parts anhydrous 
vinegar in 100 parts bone oil. The acetate and 
oleate of copper salts thus produced, form first a 
thin green layer, which hinders the attachment 
of dirt and dust, and occasions further patina 
formation.— ^^Hf/Zis/t Mechanic. 
A Screw-Driver Improvement.— Most people 
who use screw-drivers must be occasionally in- 
clined to use strong language at their persistency 
in slipping out of the nick, and their refusal to 
go into it. All that is wanted is a short tube, big 
enough to enclose the screw-head somewhat 
tightly, but only spring-tight, so that it may rise as 
the screw-head comes near the wood. Also, to pro- 
vide for heads of different sizes with the same 
driver. All this can be done by just turning up a 
strongish tin tube, throe or four inches long, like 
a slate pencil case, big enough to slide over the 
widened point of a round screw-driver, and then 
fitting spring-tight on the shank by means of a 
piece ef leather wrapped round it; and for larger 
screws than usual, you might pull the socket off 
and put a larger on, with a thicker piece of leather. 
I find it answers perfectly. You need not even 
look at your screw, but just put the tubed screw- 
driver on, and turn. It will drop into the nick at 
the first half-turn, and stay there till the screw is 
screwed home.— English Mechanic. 
Pocket Knives.— The thwytel or whittel of 
Chaucer's period was a very poor, rude instru- 
ment, consisting of a blade of bar steel fastened 
into a wooden or horn iiandle. It was used for 
cutting food, as well as for the numerous miscel- 
laneous duties which now fall to the pocket knife. 
To the whittel succeeded the Jack-Knife— the 
Jacques-de-Liege, or Jock-te-leg of the Scottish 
James VI— which formed the prototype of the 
modern clasp knife, inasmuch as the blade closed 
into a groove in the handle. This improved form 
was probably introduced into Sheffield by Prostes- 
tant refugees from the Low countries, who came 
to England during the reign of Queen Elizabeth, 
Shortly thereafter, about the beginning of the 
17th century, the pocket knife with spring back 
was introduced, and no marked improvement 
took place till the early part of the present cen- 
tury. 
