L I G 
H T. 
680 
■weather, fource of the light, &'c. has been lately noticed 
in the Papaver orienfale, or oriental poppy, as detailed 
by a correfpondent in the Monthly Magazine, vol. xxxi. 
,c ‘ Walking in my garden the other evening with a friend, 
we were both ftruck with this appearance at the fame in- 
ftant: the fparks (or flafhes rather) were very brilliant and 
in quick fucceflion, the petals at the fame time clofmg fre¬ 
quently with fudden jerks. The flafhes proceeded entirely 
f rom the infide of the flowers, and, when they were clofed, 
ceafed ; the day had been very hot and fhowery, with 
much diflant thunder, the fun was jufl fet, the evening 
calm, and the dew falling. I mention thefe circumflances, 
as I have been frequently on the watch fince for the fame 
appearances in a different (fate of the weather, without 
fuccefs.” The letter is dated June 13, 1811. 
It has been too common for chemiits to draw the fol¬ 
lowing conciuflon; that, when light or light and heat to¬ 
gether are evolved, it mull either have arifen from com¬ 
buftion, or that the light is a component part of the body 
from which it is difengaged. As, for inftance, becaufe 
Canton’s phofphorus fliines without the prefence of oxy¬ 
gen, the light is called light of combination ; and Dr. Hulme 
ba.s, with lefs foundation, drawn the fame conciuflon. In- 
ilead of faying that light and heat are products of combi¬ 
nation from the union of oxygen with inflammable mat¬ 
ter, we fliould fay that it is the refult of rapid chemical 
combination when the bodies have great affinity for each 
other. We have feveral fa< 5 ls which confirm this idea. 
When ftrong mineral acids combine with pure potafh, 
lime, or magnefia, much heat and foine light are emitted. 
The fame thing is alfo obferved in flacking of lime. In 
an experiment made by a fociety of chemifts it appears, 
that, when a mixture of fulphur and copper-filings is ex- 
pofed to a red heat, in a glafs tube-, the oxygen being ex¬ 
cluded, the two fubllances fuddenly combine, attended 
with the difengagement of light. In thofe chemical 
changes where heat and light are difengaged, the follow¬ 
ing law will obtain : The change of temperature will be 
as the difference between the fpecific heat of the com¬ 
pound body and half the fum of the fpecific heat of the 
bodies before combination ; while the intenfity of the light 
and heat will be inverfely as the tihie in which this change 
has been taking place. 
III. We fliall here leave the fubjeft of phofphorefcent 
light to give fome account of that produced by coni- 
bufiion ; in treating which, we fliall find our progrefs 
much facilitated, by confidering combuftion as dependent 
on the above law, rather than upon the laws of com- 
buftion as laid down by Lavoifi.er, who was of opinion, 
•that the light and heat furnifhed by combuftion were en¬ 
tirely derived from the oxygen. See our article Che¬ 
mistry, vol. iv. p. 195, 6. If, as we have fuppofed, the 
quantity of heat be greater as the fpecific heat of the re- 
fulting compound is lefs than the mean of the bodies be¬ 
fore combination, we ought to have heat evolved when¬ 
ever fuch change can be proved ; and, by afeertaining be¬ 
fore-hand the fpecific heat of the compound and of the 
elements, the quantity of heat may be known. Experi¬ 
ence has already given great ftrength to this notion. The 
intenfity of the light and heat, however, during thefe 
changes, will not depend upon the abfolute quantity 
evolved, but upon the rapidity of the evolution ; and, if 
we are not greatly deceived, the quantity of light will al¬ 
ways be as the rapidity of combuftion. In the flow corn- 
bullion of hydrogen gas, the light is not great, but the 
whole heat is greater than that afforded by any other 
combuftible body. On the other hand, the abfolute 
quantity of heat afforded by the combuftion of phof- 
pliorus is much lefs than that evolved by burning an 
equal weight of hydrogen ; but the quantity of light 
given by the former much exceeds that of the latter. 
The intenfity of light, however, will alio be inverfely as 
the (pace which it occupies; and hence it will be as the 
fpecific gravity of the combuftible body. We may there¬ 
fore conclude, that the quantity of light afforded by 
combuftion will be as the rapidity of combuftion, which 
will be as the affinity of the body for oxygen, as the den- 
fity of the burning body, and inverfely as the coliefion of 
the body. The difference of cohefion between charcoal 
and the diamond accounts for the relative combuftibility of 
thefe two bodies. For this reafon, foft iron wire ought to 
afford more vivid combuftion in oxygen than ileel wire. 
In order to obtain a relative idea of the value of dif¬ 
ferent combuftible bodies ufed for procuring artificial 
light, we fhall detail fome ingenious experiments made by 
Dr. Henry, and publilhed in Nicholfon’s Journal, vol. xi. 
Dr. Henry, with a view to afeertain the relative value 
of the combuftible gafes, made fome trials with hydrogen, 
carburetted hydrogen, and carbonic oxyd. Thefe he 
found did but afford /a very inferior light, compared with 
the fplendid light given by the gas afforded by the de- 
ftruflive diftillation of pit-coal, which is equal to the 
light given by the fineft fpermaceti oil. The following 
Table points out the refult of his experiments, and clearly 
finows the caufe of the fuperior property of coal-gas to 
produce light. 
Kind of Gas. 
Meafures of Oxygen 
Gas required to fatu- 
vate one hundred inea- 
fures of each. 
Meafures of Carbo-J 
uic Acid produced. 
Pure hydrogen 
50 to 54 
None 
Gas from nioift coal 
60 
35 
Do. wood (oak) 
54 
33 
Do. dried peat 
68 
43 
Do. from cannel coal 
I 70 
IOO 
Do. lamp oil 
I 90 
124 
Do. wax 
220 
137 
Pure olefiant gas . | 284 
1 79 1 
The firft column contains the different gafes, 100 mea- 
fures of each being ufed in each experiment. The fecond, 
the meafures of oxygen which were confumed while each 
of the 100 meafures were burning. The third, the quan¬ 
tity in meafures of carbonic acid which refulted from the 
combuftion. It is a faft already afeertained, that every 
meafure of carbonic acid gas has refulted from a meafure 
of oxygen ; confequently, the quantity of oxygen con- 
fumed in its formation is equal in meafure to the num¬ 
bers in the third column ; the excefs of oxygen, there¬ 
fore, appearing in the fecond. By fubtraiting the num¬ 
ber in the third from that in the fecond, it will give the 
quantity of oxygen which has combined with the hydro¬ 
gen in each of the gafes. This excefs of oxygen, in the 
fecond column, will combinewith two meafures of hydro¬ 
gen, to form water. I11 order to form fome idea of the 
relative value of thefe combuftible gafes, we will compare 
the firft, which is pure hydrogen, and the laft, or the pure 
olefiant gas, which has the greateft efficacy in producing 
light. The 50 meafures of oxygen in the firft combine 
with 100 of hydrogen ; and, fince no carbonic acid is 
produced, this is the whole effeft. I11 the laft experiment, 
179 of carbonic acid is formed at the expenl’e of 179 mea- 
fures of oxygen and about ^th its weight of carbon, 
which would alone have furniffied confiderabie light. 
Since, however, 284. meafures of oxygen are expended, 
we have 28+— 179 = 105 meafures of oxygen, which 
would require 200 meafures of hydrogen. It, therefore, 
hydrogen and carbon were equally efficacious as com¬ 
buftible bodies in producing light, the quantity of light 
in one, to that in the other, would be as 210 ft-y X J79 : 
100, or as 25 to 1 nearly. The ratio of the fpecific gra¬ 
vities of tilde gafes is as 90 to 8 nearly; therefore, mul- 
• tiplying 
