470 TRANSACTLONS OF SECTION B. 
J think, a great clarification of view has taken place. The old idea that there 
existed a fixed temperature at which inflammation suddenly took place cannot 
now be maintained, and the term ‘ignition temperature’ has acquired a different 
meaning. It is now known that in a very great number of cases a mixture of 
two flame-forming gases, when gradually raised in temperature, will develop lumi- 
nosity quite gradually, part passu, with the chemical combination that is being 
induced. This phenomenon is, of course, known universally in connection with 
phosphorus, but it is not so widely known in connection with other combustible 
substances. There are some simple facts that seem as if they never could gain 
admission to text-books, and I do not think I have known more than a single 
chemical book that is not likely to leave a student under the impression that the 
phosphorescence of phosphorus is an almost unique phenomenon. J do not know 
how many times the independent discovery has been made that sulphur, arsenic, 
carbon disulphide, alcohol, ether, paraffin, and a whole host of other compounds, 
inorganic and organic, will phosphoresce as truly as phosphorus itself; that, in fact, 
phosphorescent combustion is the normal phenomenon antecedent to what we 
ordinarily call flame. 
This is, after all, only in harmony with the general truth that chemical com- 
bination between two gases does not set in suddenly, but comes into evidence 
quite gradually as the temperature is raised from a point at which the action, if it 
occurs at all, is so slow as to be negligible. The increase in the rate of combina- 
tion is, of course, very rapid as compared with the increase of temperature, a 
difference of about 10° C. serving to double it. The interval between the 
beginning of phosphorescence and the production of vigorous flames may there- 
fore be very short. In the case of phosphorus this interval, being from 7° to 
to 60° C., includes ordinary atmospheric temperatures ; hence the phosphorescence 
of phosphorus is a phenomenon that could not well be overlooked. If the pre- 
vailing terrestrial temperature were below 7° C., at which, under normal air- 
pressure, the phosphorescence of phosphorus ceases, it is possible that this element 
might never have acquired its peculiar reputation; it would not have shone in 
the dark, and in lighting it with a taper the phosphorescent interval would have 
been passed over as quickly as is ordinarily the case in the ignition of sulphur, 
paraffin, and other common combustibles. To make phosphorescence apparent in 
these last cases it is necessary to take special care to heat up a mixture of the 
combustible gas and air gently, and to maintain it at a temperature approaching, 
but not quite reaching, that of ignition. There is no simpler way than that 
used by Sir William Perkin, who brought the combustible substance near to, 
or in contact with, a massive metal ball previously heated to the suitable 
temperature, 
The change from phosphorescence to ordinary flame is not sudden, but the 
appearance of ordinary flame is the end point of a continuous, though rapid, develop- 
ment. This end point is the temperature of ignition, What, then, determines 
the temperature of ignition? The answer to this question has been given with 
characteristic conciseness by Van't Hoff as ‘ the temperature at which the initial 
loss of heat due to conduction, &c.,, is equal to the heat evolved in the same time 
by the chemical reaction.’ 
We may obtain a clear idea of the meaning of temperature of ignition by 
supposing a combustible mixture of gases such as that of air and the vapour of 
carbon disulphide to issue through an orifice into an indifferent atmosphere. If we 
surround the orifice by a ring of platinum wire, which is gradually heated up by 
a current of electricity, a flame will gradually make its appearance. If, as soon 
as this is observed, the heating of the wire by the current be discontinued, the 
flame will disappear; it is, in fact, not self-supporting, but depends on the 
accessory supply of heat through the electrically heated wire. If now we raise 
the rmg to a higher temperature we shall get a brighter flame, owing to an 
increased rate of chemical action, and at last we sball reach a point where it is 
possible to cut off the electric current without causing at the same time the 
extinction of the flame. This is the true temperature of ignition, the temperature 
at which the reaction proceeds st a rate just sufficient to overbalance the loss of 
