%ht papular Science 0tMiQ 
AND 
BOSTON JOURNAL OF CHEMISTRY. 
Volume XXV. 
BOSTON, MAY, 1891. 
NUMBEE 5. 
CONTENTS. 
Familiar Science.— Platinum Mirrors . . 63 
ITie Balanced Rock of Tandil ...'.. 63 
Studies in Plant Biology 64 
Geological Disturbances and Climatic 
Changes 65 
Effects of Pressure on Ice 66 
A Wonderful Artist 66 
A Convenient Method for Detecting and Es- 
timating Petroleum in Spirits of Turpen- 
tine 67 
An t;xperiment for Showing Diffusion of 
Liquids 67 
Prevention Societies 67 
Scientific Brevities 67 
[The Out-Door World.— Our Official Organ 68 
Selected Reports from the First Century 
(Chapters 1-100) 68 
Another Microscope Prize 70 
An Important Worii 70 
List of the Plants of the District of Mos- 
chaisk, Government of Moscow, Russia . 70 
Editorial.— What We Owe to Gas Works . 71 
" Impressionism " in Photography ... 72 
Earth-Stars 72 
The Phienicians in Britain 73 
Meteorology for March, 1891 78 
Astronomical Phenomena for May, 1891 . . 74 
Questions and Answers 74 
Literary Notes 74 
rMEDiciNE AND PHARMACY.— The Faith Cure 
in Ancient Greece 75 
Beauty as a Means of Health ...... 76 
The Rational Treatment of Pneumonia . . 76 
A Convenient Filter 77 
Monthly Summary of Medical Progress . . 77 
Medical Miscellany 78 
(Publishers' Column '^•. 78 
familiar ^e'lenee. 
PLATINUM MIRRORS. 
Nearly all the mirrors in common use are pro- 
, vided with a reflecting surface consisting of an 
\ amalgam of tin and mercury. The manufacture 
' of these mirrors is not only injurious to the health 
J of the workmen, but the coating is very liable to 
|4njury by accident or by the spontaneous crystal- 
l lization and decomposition often observed in old 
■' miners or those exposed to strong sunlight. 
As it is a comparatively easy matter to precipi- 
itate upon a plate of glass a coating of metallic 
^gold, silver, or platinum, many attempts have 
; been made to use these metals as a substitute for 
|he mercury-tin amalgam; but none of the mir- 
Ors thus produced are quite as satisfactory or 
"brilliant as the latter. Recently a French firm — 
the MM. Dode — has produced platinized mirrors 
which are not only satisfactory in point of bril- 
liancy, but are as permanent and unchangeable as 
the glass itself. A thin film of metallic platinum 
is deposited upon the glass, by precipitation from 
a solution of the chloride, by oil of lavender, and 
afterwards burnt in by heating the glass nearly to 
redness in a muffle, thus incorporating the reflect- 
ing surface into the glass itself. 
A peculiarity of this coating of platinum is its 
extreme thinness, which readily allows light to be 
transmitted through it. Thus if the light falls 
Jbig. 1. 
upon the mirror from the front, it will be reflected 
hack again the same as in an ordinary mirror ; but 
if the source of the light is behind the plate of 
platinized glass, the rays will be transmitted to 
the observer standing before it as if it was a plate 
chamber, containing a grotesque head, and which 
can be illuminated or darkened at pleasure. In 
Fig. 1 the light behind the mirror is extinguished, 
and the person looking into it sees only his own 
reflection. If, now, the chamber in the rear of 
the glass is illuminated, the reflected image dis- 
appears, and the victim of the scientific joke sees 
instead of his own features the less flattering 
image depicted in Fig. 2. 
Under favorable conditions ordinary plate-glass 
without any metallic coating will reflect nearly all 
the light falling upon it, and by a similar arrange- 
ment of alternately illuminating the front and 
rear of such a glass most of the "ghosts" and 
other mysterious illusions of the stage are pro- 
duced. 
THE BALANCED ROCK OF TANDIL. 
The so-called "balanced rocks" are not very 
uncommon, and are found in nearly all countries. 
Some of these are of glacial origin, having been 
left in their peculiar position by the melting of 
the glacier which brought them from some dis- 
tant locality, while others have becin carved out 
of ordinary glass. The illustrations (from La Na- 
ture) show an amusing application of this princi- 
ple. The mirror is placed in front of a niche, or 
mg. a. 
on the spot l)y the destructive action of the ele- 
ments, especially by the erosive effect of sand 
blown by the wind, this natural sand-blast hav- 
ing an almost Irresistible power. The strangely 
carved rock formations of Colorado and the Rocky 
Mountains are due to this cause, while the glacier- 
borne boulders are, more common in New England 
and sections of the country lying north of the 
Ohio River. 
The remarkable example of a balanced rock 
shown in the engraving (Fig. 1) occurs at Tandil, 
South America, near Buenos Ayres, and the illus- 
tration wiis engraved from a photograph for La 
Nature, from which we reproduce it. From the 
original description it would appear that the rock 
was carved out on the spot by the action of the 
elements, although it would seem from the en- 
graving that it must have been originally brought 
and deposited on the spot by some ancient glacier. 
The large rounded boulder shown in Fig. 2 is close 
by, and indicates even more strongly the action of 
ice or glaciers. 
