SCIENTIFIC SUMMARY. 
135 
and the metal employed. In the liquids, it gives three distinct spectra — 
one of the metal, one of the vapour of the liquid, and, after some time, 
one of solid incandescent carbon. The spectra produced in gases are 
never very brilliant. The constitution of the electral spark is invariably 
the same, no matter what be the condition of the medium in which it is 
produced, nor whether it be liquid or gaseous. We have, generally — 1st, 
volatilization of the polar metal , and incandescence of the vapour produced ; 
2nd, incandescence of the elements of the medium through which the current 
passes. In certain cases, the medium alone or some one of its elements 
appears to become incandescent ; in other instances, the vapour of the 
metal is the luminous substance. All simple bodies which have a great 
affinity for the metals, whether they are free or in combinations easily 
decomposed, intensify the metallic spectrum. 
Electrical Conductibility of Thallium . — The conductibility of this metal 
has been estimated, according to Wheatstone’s method, by M. de la Rive. 
It is expressed by the formula — 
n- l,D 
° WE 
when L = length of the wire ; D, the density ; W, the weight ; and R, 
the resistance. Four wires of various lengths were used, and the mean 
figure for conductibility was calculated at 5*30, as compared with mercury. 
Absorption of Heat by Gases. — Professor John Tyndall delivered a lecture 
some time since in the Royal Institution, in which he showed that certain 
gases have the power of intercepting radiant heat to a far greater extent 
than has been supposed. A very simple apparatus for the purpose of 
determining this capacity on the part of gases, was exhibited. It consists 
of a hollow cylinder, closed at one extremity by a plate of rock-salt, into 
which was fitted a second and smaller cylinder closed in a similar manner. 
As the second moved piston fashion within the first, it is evident that by 
enclosing the gaseous matter between the two plates, a layer of any required 
tenuity might be obtained. The lecturer performed several beautiful 
experiments, which illustrated his views in the most striking manner. It 
was shown that a layer of olefiant gas, of a thickness of two inches, inter- 
cepts about thirty per cent, of the entire radiation. If our earth were 
surrounded by such a layer, the temperature would be something fearful 
to contemplate ; for, though it would offer no decided obstruction to the 
passage of the sun’s rays, it would intercept and, to a great extent, return 
thirty per cent, of the earth’s radiation. It was also demonstrated that 
both absorption and radiation are molecular phenomena, and are not 
dependent on the aggregation of the molecules. If a certain vapour be a 
powerful radiator and absorber, the liquid from which it proceeded has 
similarly intense properties. The order of absorption in liquids and 
vapours is exactly the same. 
The Specific Heat of Solid Bodies. — A memoir enunciating new views on 
this subject has been presented to the French'" Academy by M. H. Kopp. 
He maintains — 1st, that every element in the solid state, and at a certain 
number of degrees from the fusing-point, has a single specific heat, and 
consequently a single atomic heat also ; 2nd, that this specific heat is 
subject to certain variations, according to the physical conditions of the 
