260 
POPULAR SCIENCE REVIEW. 
of pure water, and thus all those portions not covered with ink are allowed 
to absorb the liquid. It is then put between two sheets of blotting-paper, 
which carry off the excess of water ; after which it is laid face downwards 
on the stone, to which it adheres perfectly. Another sheet is laid on this, 
and moistened with dilute nitric acid ; the acid penetrates both sheets, and 
eats away the stone in accordance with the lights and shades of the original 
picture. 
Improvements in the Barometer. — Some important improvements have 
recently been effected in the Aneroid barometer by Messrs. Cook & Sons, the 
opticians. Although the Aneroid, under ordinary circumstances, has been 
shown by Mr. Glaisher and others to be very much more effective and satis- 
factory in its results than could have been hoped, still, under conditions which 
bring rapid changes of pressure into play, the instrument when it returns to 
the normal pressure does not always indicate correctly. This results from 
the motion being communicated to the index axle by a chain, and this chain, 
from other considerations, is the weakest part of the instrument, and is the 
first acted upon by climatic influences, rust, &c. Mr. Cook has abolished 
this chain altogether, substituting for it an almost invisible driving-band of 
gold or platinum, and the result of this great improvement is that the Aneroid 
may now be looked upon as an almost perfect instrument for scientific re- 
search. Several such Aneroids, placed under the receiver of an air-pump, 
not only march absolutely together, but all return unfailingly to one and the 
same indication. — Vide The Header, March. 
The History of the Discovery of Spectrum-analysis . — A paper has on this 
subject been presented to the French Academy by Sir David Brewster, 
who thinks that the labours of those who were first in the field of discovery 
have been seriously overlooked. He considers that the labours of Mr. Fox- 
Talbot were prior to those of M. Tantedeschi. As early as 1826, Dr. David 
Brewster received from Mr. Talbot an essay on some “ experiments on 
coloured flames,” in which he says that the orange tint may be due to 
strontian, and that if such were the case, a glance at the prismatic spectrum 
might indicate that it contains substances which otherwise could only be 
discovered .by laborious analysis. M. Volpecelli, in 1863, declared that Sir 
David had made “ many discoveries in spectral analysis, which have been 
actually developed with modern spectroscopes.” As a proof that this assertion 
was well founded, certain passages from the writings of Sir David Brewster 
were adduced by him. One of these, written in 1833, states that “the 
principal object of his researches had been the discovery of a general principle 
of chemical analysis, in which both simple and compound bodies would be 
characterized by their action on definite portions of the spectrum and it 
was remarked, at the same time, “ that those very absorbing elements which 
exist in nitrous acid gas are found likewise in the atmospheres of the sun 
and earth.” In 1842, Sir David discovered that luminous and brilliant lines 
in certain flames correspond to the lines which are wanting in solar light ; 
and a series of experiments made by him on 180 different substances, which 
were ignited in a platinum cup, by means of a mixture of oxygen and hydrogen, 
showed that this is “ a property which belongs to almost every flame.” The 
places of the lines were, in these experiments, merely estimated by the eye ; 
but it was anticipated at the time that other experimentalists would deter- 
