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Aug. 18, 1870| - 
NATURE 
siey. 
and a second or even third blow is therefore required to determine 
its explosion. 
These circumstances would appear to afford support for the 
belief that the detonation of an explosive material through the 
agency of a blowis the result of the development of heat 
sufficient to establish energetic chemical change, by the ex- 
enditure of force in the compression of the material, or by the 
riction of the particles against each other, consequent upon a 
motion being momentarily imparted to them. It is conceivable 
that, from either of these causes, sufficient heat may be accumu- 
lated with almost instantaneous rapidity, in some portion of the 
mass struck, to develop sudden chemical change. ‘The cir- 
cumstance that the detonation of those portions of an explosive 
compound (such as gun-cotton or nitroglycerine) which are 
immediately between the surfaces of the hammer and the sup- 
port is not communicated to the surrounding portions, may be 
ascribed to a combination of two causes, the instantaneous nature 
of the explosion, and the close confinement of the portions struck 
at the instance of their explosion, The mechanical effect of the 
detonation is absorbed by the masses of metal between which it 
occurred, and the gases developed disperse the surrounding por- 
tions of the explosive agent, as they rush away from between the 
two surfaces. It is possible also to detonate gunpowder and 
other explosive mixtures by a blow in such a manner that only 
the portions immediately struck are ignited; but those sub- 
stances may also be exploded, though much less violently, by 
a less sudden or powerful application of force, in which case 
they detonate much more feebly ; their explosion is accom- 
panied by a larger volume of flame, and by the ignition of 
those portions which surround the part struck by the hammer. 
The power of accomplishing the explosion or detonation of gun- 
cotton or nitroglycerine in open air through the agency of a 
detonation produced in its vicinity, would therefore appear to be 
correctly ascribable to the heat suddenly developed in some 
portion of the mass by the mechanical effect, or blow, exerted 
by that detonation, and would seem to be regulated by the 
violence and suddenness (either singly or combined) of the deto- 
nation, by the extent to which the explosive material is in a 
condition to oppose resistance to the force, and by the degree of 
sensitiveness of the substance to explosion by percussion. There 
are, however, several well-known facts, and some results of 
experiments instituted with special reference to this subject, 
which do not appear to be in harmony with the assumption that 
the detonation of nitroglycerine and gun-cotton in the manner 
described is simply due to the suddenness of the development and 
application of physical force. 
With the view of ascertaining whether the relative power of 
different explosive agents to accomplish the detonation of gun- 
cottonappears to be in direct proportion to the relative mechanical 
effects of their explosion (/.e. to the work performed by them 
upon a body placed in contact with them), a series of experiments 
was instituted with the object of comparing this particular action 
of the several explosive materials. It would appear from these 
experiments that, when unconfined, the violence of explosion of 
chloride of nitrogen is less than that of the iodide, and that, if 
confined under water, it very considerably exceeds that of the 
exposed iodide, but falls very short of that exerted by unconfined 
silver-fulminate. It also appears that the mercuric fulminate, 
which is much less rapidly explosive than either of the other sub- 
stances, exerts less mechanical force than any of them, if freely 
open to air, and if inflamed at some portion of the exposed sur- 
faces; if ignited at the lower inner portion of the mass, where 
the part first inflamed is enclosed by the mass of the material 
itself, it exerts a destructive force little inferior to that of the 
chloride of nitrogen enclosed by water; but if confined in a 
strong envelope (e.g. of sheet tin), the mercuric fulminate is 
greater in violence of action than the unconfined silver-fulminate. 
These results to a great extent confirm the correctness of the view 
that the readiness with which the detonation of gun-cotton is 
accomplished is in proportion to the mechanical force exerted by 
the initiative detonation to which it is subjected. The force 
exerted by small quantities of strongly confined silver and mer- 
curic fulminate greatly exceeds that developed by the explosion 
of comparatively large proportions of the iodide and chloride of 
nitrogen. “This may be accepted as accounting, to some extent, 
for the fact that the detonation of gun-cotton could not be 
accomplished by an amount of iodide of nitrogen twenty times 
greater than that of fulminates required for the purpose, while 
ten times the quantity of the confined chloride were required to 
produce the result. That the quantity of mercuric fulminate 
required to produce detonation is reduced in proportion as means 
are applied to increase the violence of the force exerted by it at 
one time, is quite in accordance with the aboye view. 
I venture to offer the following as being the most satisfactory 
explanation which occurs to me of the remarkable differences 
exhibited in the behaviour of different explosive agents. The 
vibrations produced by a particular explosion, if synchronous with 
those which would result from the explosion of a neighbouring 
substance which is ina state of high chemical tension, will, by 
their tendency to develop those vibrations, either determine the 
explosion of that substance, or at any rate greatly aid the dis- 
turbing effect of mechanical force suddenly applied, while, in the 
case of another explosion which produces vibrations of different 
character, the mechanical force applied by its agency has to 
operate with little or no aid ; greater force or a more powerful 
detonation must, therefore, be applied in the latter instance, if 
the explosion of the same substance is to be accomplished by it. 
In conclusion, it may not be out of place to refer briefly to a 
few illustrations of the important bearings which the new mode 
of developing the explosive force of gun-cotton has upon the 
practical uses of the material as a destructive agent. The con- 
finement of a charge of gun-powder or gun-cotton in a blast- 
hole, by firmly closing up the latter with earth, powdered rock, 
or other compressible material (by the process known as tamp- 
ing or stemming) to a depth greater than the line of least resist- 
ance opposed to the action of the charge, is essential to the suc- 
cess of a blasting operation; but the great rapidity of explosion, 
by detonation, of a charge of gun-cotton greatly reduces the value 
of this operation ; the destructive effect of the material, when 
exploded in a hole which is left open, is not inferior in extent to 
that obtained by similarly exploding a charge confined in the 
usual manner. Thus the most dangerous operation in connection 
with blasting may be entirely dispensed with. In submarine 
operations, it is no longer necessary to enclose the charge of ex- 
plosive agent in the strong and therefore cumbersome metal re- 
ceptacles hitherto required to ensure the full development of its 
explosive force ; the destructive action of a charge of gun-cotton, 
enclosed in a waterproof bag or thin glass vessel and exploded 
by detonation, being decidedly greater than that furnished by a 
corresponding charge confined in a strong iron vessel and ex- 
ploded by flame. Small charges of gun-cotton simply resting 
upon the upper surfaces, or loosely inserted into natural cavities, 
of very large masses of the hardest description of rock or of iron, 
have broken these up as effectually as if corresponding charges 
had been firmly imbedded in the centre of the mass and ex- 
ploded in the usual manner. Lastly, the certainty, facility, and 
expedition with which certain important military destructive 
operations may be accomplished by means of gun-cotton exploded 
by detonation, are not among the least important advantages 
which are now secured to this interesting and remarkable explo- 
sive agent. 
SCIENTIFIC SERIALS 
PoGGENDORFF’S Annalen der Chemie und Pharmacie, vol. cxl. 
part 1.—This number contains (1) the first part of an elaborate 
paper by E. Ketteler, ‘‘On the Influences of Ponderable 
Molecules on the Dispersion of Light, and on the Signification 
of the Constants in the Mathematical Formule for Dispersion ” 
(pp. I to 53). ‘This is a critical examination, based chiefly on 
Mascart’s experimental measurements, of the formula: by which 
Cauchy and others have endeavoured to connect the indices of 
refraction of the various kinds of light with their wave-lengths. 
The nature and scope of the investigation may be gathered from 
the four following criteria which the author gives as the tests of 
a satisfactory formula :—‘‘1. A rational formula must enable 
us to calculate accurately from their wave-lengths the succession 
of the several colours and their distribution in space, for the 
whole measured extent of radiation, for some definite density of 
the dispersive medium. 2. The constants of the formula must 
be capable of a distinct physical interpretation, analogous to 
the interpretation assigned by Cristoffel to the constants in his 
formula. 3. When the density of the dispersive medium is 
altered, these constants must participate in the change of molecular 
constitution in some simple manner, corresponding to what has 
been ascertained in respect to them in the case of gaseous media. 
4. Consequently, as the medium approaches the limit of rare- 
faction, all the indices must approach unity as their limiting 
value.” The author finds that none of the formulz hitherto 
proposed reproduce the experimental results within the limits ot 
