64 
for foundry work ; that the ill-effects of one constituent can at 
best be only imperfectly neutralised by the addition of another 
constituent ; that there is a suitable proportion for each consti- 
tuent present in cast-iron, depending upon the character of the 
product desired, and upon the proportion of other elements 
present ; and that variations in the proportion of silicon afford a 
reliable and inexpensive means of producing a cast-iron of any 
required mechanical character which is possible wich the material 
employed. 
Krupp’s hot-blast pyrometer, which wa; shortly described, 
consists of an arrangement by which the hot blast is drawn with 
a fixed proportion of cold air into a chamber, the temperature 
of which, being measured with an ordinary thermometer, gives 
that of the hot blast by calculation, 
ON DISSOCIATION TEMPERATURES, WITH 
SPECIAL REFERENCE TO PYROTECHNICAL 
QUESTIONS? 
IN bringing the subject of dissociation before the Reyal Insti- 
tution of Great Britain, the author proposed to confine 
himself to its influence on combustion and heating, that is to 
say, to its effects on combustible gases and the products of com- 
bustion, and on furnace work generally. His researchés had 
been made for the most part in connection with large gas fur- 
naces constructed according to his new system of working with 
radiated heat, or what may be otherwise called free development 
of flame. In the first or active stage of combustion the flame 
passed through a large combustion chamber (all contact with its 
surfaces being avoided), and parted with its heat by radiation 
only ; while in its second stage the products of combustion were 
brought into direct contact with the surfaces and materials to be 
heated, by which means the remainder of its heat was abs- 
tracted. This, in a few words, was a description of the method 
of heating with free development of flame. In perfecting this 
system of furnace, the principle of which was in many respects 
the reverse of that generally accepted, both as regards construc- 
tion and working, he had to examine into the accuracy of certain 
scientific theories which could not be brought into harmony with 
the actual results he obtained. 
Adopting the generally-accepted theory of combustion, accord- 
ing to which a flame consists of a chemically-excited mixture of 
gases, whose particles are in violent motion, either oscillating to 
and from each other, or rotating around one another, it followed 
that any solid substance biought into contact with gases, thus 
agitated, must necessarily have an impeding effect on their 
motion. Motion being the primary condition of combustion, 
the latter would be more or less interfered with, according to 
the greater or less extent of the surfaces which impede the 
action of the particles forming the flame; in the immediate 
neighbourhood of such surfaces the combustion of the gases 
would cease altogether, because the attractive influence of the 
surfaces would entirely prevent their motion; farther off, their 
combustion would be partial, and only at a comparatively great 
distance the particles of gas would be free to continue unim- 
peded the motion required to maintain combustion. On the 
other hand, the surfaces themselves must suffer from the motion 
of the particles of gas producing the flame, for, however small these 
particles might be, they produce, while in such violent motion, an 
amount of energy which acting constantly would in time destroy 
the surfaces opposed to them, just as ‘‘ continual dropping wears 
away stone.” This circumstance fully accounted for the fact 
that the inner sides of furnaces, and the materials they contained 
were soon destroyed, not by heat, but by the mechanical, and 
perhaps also by the chemical, action of the flame. It would 
seem strange that the heating power of a large volume of flame 
should be so much interfered with by the contact of its outer 
parts only with the inner sides of a large furnace chamber, if 
there was not another cause besides imperfect combustion to 
reduce the heating effect of a flame which touched the surfaces 
to be heated. 
heat not only from its outer surface, but also from its interior by 
allowing the heat to radiate through its mass. In this manner 
every particle of flame sent its rays in all directions, but if the 
flame itself touched anywhere combustion ceased there, free 
carbon was liberated and produced smoke which enveloped that 
part and prevented the rays of heat of the other portions of the 
flame from reaching it. 
ee Lecture by Mr. Frederick Siemens at the Royal Institution, Friday, 
ay 7. 
NATURE 
A flame when in a state of combustion radiated | 
[May 20, 1886 
The author had avoided for various reasons referring to the: 
subject of dissociation until recently, although it had been 
brought forward by several writers, and used as an argument 
against his new system of furnace ; as according to these writers’ 
it would appear to be impossible to produce such exceedingly- 
high temperatures as he claimed to reach. He had long held 
the opinion that appearances of dissociation not being observ- 
able in furnaces heated by radiation, but occurring in furnaces 
in which the flame was allowed to come into contact with 
surfaces, must be due to the action on the flame of those sur- 
faces at high temperature. He was led to this conclusion partly 
from his own observations, and partly from descriptions of dis- 
sociation observed by others, amongst whom was his brother, 
the late Sir William Siemens, who described a case of dissocia- 
tion (see lecture delivered March 3, 1879, at the Royal United’ 
Service Institution, entitled ‘*On the Production of Steel, and’ 
its Application to Military Purposes”) which occurred in a 
regenerative gas furnace constructed according to their old views 
of combustion and heating. Zhe conclusion at which he hat 
arrived was, that solid surfaces, besides obstructing active com- 
bustion, must also at high temperatures have a dissociating 
influence on the products of combustion. ; 
In order to obtain information on this subject he examined the 
laws and theory of dissociation, and endeavoured to bring the 
various results obtained by scientific authorities into agreement 
with one another, and with his own experience, but failed en- 
tirely in doing so. The temperatures of dissociation of carbonic 
acid and steam, the two principal gases forming the products of 
combustion when ordinary fuel was used, vary very much 
according to these observers, and the results he had obtained 
in practice were different from most of them. He hoped to 
prove that the temperature at which dissociation sets in is, in 
most cases, much higher than generally admitted ; and that the 
authorities he was about to refer to had omitted in almost all the 
experiments they had made to take into proper consideration 
one element which was liable to alter materially the results 
obtained by them. This element was the apparatus used for 
those exp_riments as regards its surface, form, and material, 
In considering the question of dissociation, he proposed to- 
commence with Deville, who first discovered and called atten- 
tion to the dissociation of gases at high temperatures. He made 
numerous experiments with various gases, and fixed certain tem- 
peratures at which he found that either complete or partial dis- 
sociation took place. Without going into details, he might 
mention that Deville required to use vessels and tubes of definite 
dimensions, material, and structure, in order to obtain the results 
stated. One experiment had to be made with a porous tube, 
another required the use of a vessel with rough interior surfaces, 
or containing some rough or smooth material. In this way 
Deville arrived at a great variety of results, and although he did 
not state that the rough surfaces, or porous tubes, or the solid 
material placed inside the vessels which he employed, had any 
particular influence on the temperature at which dissociation 
took place, yet it would appear that he could not obtain his 
results without having recourse to those means. Deville’s results 
depended very much upon the various kinds of surfaces he used 
in his experiments, if they were not entirely brought about by 
them ; these experiments, moreover, were of a very complicated 
nature, so he proposed to pass on to more modern authorities, 
whose experiments were of simpler character, and less open to 
objection. 
The most important experiments which modified those of — 
Deville were due to Bunsen. Bunsen observed the dissociation 
of steam and carbonic acid by employing small tubes filled with 
an explosive mixture of these gases, to which suitable pressure- 
gauges were attached. On igniting the gaseous mixture, ex- 
plosion took place, and a high momentary pressure was produced 
within the tube ; from the pressure developed, Bunsen calculated 
tho temperature at which the explosion took place, and found 
that it varied with the mixtures employed. He records the 
circumstance that only about one-third of the combustible gases 
took part in the explosion, from which circumstance he con- 
cluded that the temperature attained was the limit at which 
combustion occurred. To prove this, Bunsen allowed the gases 
sufficient time to cool, after which a second explosion was pro- 
duced, and even a third explosion when time was allowed for 
the gases to cool down again. Bunsen obtained much higher 
temperatures fr his limits of dissociation than other physicists ; 
these were for steam about 2400° C., and for carbonic acid 
about 3000° C. These temperatures were probably higher than 
