PRESIDENTIAL ADDRESS, A471 
heat by radiation, conduction, and convection from the burning layer of gases, so 
that the next layer is put in the same state, and steady combustion proceeds, 
Phosphorescence has been spoken of as degraded combustion, and, though 
literally the appellation is correct, I think it is liable to be misunderstood. 
Again, it is often supposed that phosphorescence is necessarily associated with 
the formation of incompletely oxidised products. This may be the case in a 
chemical system which is capable of affording different products at different 
temperatures, but it is not an essential feature ; the phosphorescent combustion of 
sulphur, for example, affords nothing but sulphur dioxide. 
Temperature of ignition is, then, neither a temperature at which combination 
suddenly begins nor one dependent solely on the nature of the combining gases. It 
will vary with the proportion in which the gases are mixed and with their pressure 
and other circumstances. Notwithstanding the simplicity of this conception, it must 
be admitted that there are many obscure facts connected with the ignition of gases, 
The inflammability of gaseous mixtures is not necessarily greatest when they are 
mixed in the proportions theoretically required for complete combination ; the 
influence cf foreign gases does not appear to follow any simple law ; the presence 
of a very small quantity of a foreign gas may exercise a profound influence on 
the ignition temperature as in the case of the addition of ethylene to hydrogen. 
When a mixture of methane and air is raised to its ignition temperature, a 
sensible interval (about 10 seconds) elapses before inflammation occurs. These 
facts are cognate to others which have increased upon us so abundantly in connec- 
tion with the influence of moisture on chemical change. The study of the 
oxidation of phospherus in particular brings us among rocks and shoals. Apart 
from the influence of moisture on the combination we have the limitation of the 
process by a certain tension of oxygen and by minute quantities of a vast number 
of chemical substances, among which, in spite of much labour, no other common 
bond can be found. We do not know what oxide is initially formed in the 
oxidation, and the existence of the oxides P,O and P,O is as confidently disputed 
as it is affirmed. There is some reason for believing’that the phosphorescence 
connected with phosphorus succeeds the formation of one oxide and accompanies 
the formation of another. The state of the oxygen, whether atomic, ionic, or 
molecular, which acts on phosphorus, the induced oxidation of other substances, 
the ionisation of air accompanying the oxidation—these are all matters concern- 
ing which there exists a bewildering literature that hangs over us like a cloud, 
The whole of my Address would, in fact, not suffice for a summary of the state of 
our ignorance about the oxidation of phosphorus. The subject, simple as it 
appears at first sight, is really involved with a vast number of unsolved chemical 
problems the elucidation of which would throw much light on chemical action in 
general. I may, perhaps, bequeath the topic to some successor in this Chair as 
one which may serve to illustrate the advance of knowledge since these present 
days of darkness. 
The structure of flames has always been regarded as dependent upon the 
chemical changes taking place in the differentiated regions, but until recent 
times little attention has been given to any question beyond the cause of the 
bright luminosity of hydrocarbon flames. In a flame such as that of hydrogen 
or carbon monoxide, where we have some reason to suppose that the same kind of 
chemical transaction is taking place throughout the region of combustion we should 
not expect to find a differentiation of structure, and, as a matter of fact, we do 
not find any. Erroneous ideas have gained currency from the use of impure 
gases, and hydrogen is still described as burning with a pale blue flame, although 
Stas long ago stated that if the gas is highly purified, and the air freed from dust, 
the flame even in a dark-room can only be discovered by feeling for it ; a fact con- 
sistent with the line spectrum of water lying wholly in the ultra-violet. The pre- 
sence of a very small quantity of free oxygen in carbon monoxide destroys the perfect 
simplicity of the single shell of blue flame with which the purified gas burns, and 
in other flames small quantities of gaseous impurities or of atmospheric dust give 
rise to features of structure and halos which have been frequently supposed to 
pertain to the flame of the combining gases. The fringe of a flame in air may be 
often tinged by the presence of oxides of nitrogen, __ 
