890 PBOPESSOR BIOTSEN AjS'D DE. H. E. EOSCOE’S PHOTO-CHE^ITCAE EESEAECHES. 
evolved, that 1’95 cubic centimetre of hydrogen and chlorine can be thereby combined 
as hydrochloric acid. 
The mean composition by volume of the coal-gas burnt in the flame is, however,— 
Hydrogen 41-42 
Marsh-gas 39-49 
Carbonic oxide 5 -97 
Elayl 4-57 
Ditetryl 3-25 
Nitrogen 5-10 
Carbonic acid 0-20 
100-00 
In the combustion of one cubic centimetre of this gas, an amount of heat is evolved 
sufiicient to raise 1 grm. of water from 0° to 6°-8 C. ; one cubic centimetre of hydrogen, 
on the other hand, on combining with chlorine, evolves heat enough to raise 1 gi-m. of 
water from 0° to 2°-08 C . ; hence we may conclude that for every thermal unit evolved 
by the union of the components of the coal-gas flame in question -with the oxygen of the 
ah’, there are only 0-30 thermal units evolved from the chemical combination effected 
by this flame upon an infinitely extended atmosphere of chlorine and hydrogen. 
When it is required to represent in absolute measure the amount of light which falls 
during a time t on the unit of surface, it is most convenient to express the photo-chemical 
action of the rays falling on such a surface according to formula (4.), as a column of 
hydrochloric acid at 0® C. and 0’76, which would have been produced if the light had 
passed in a parallel direction through an unlimited atmosphere of chlorine and hydro- 
gen. This light, measured in metres, we propose, for the sake of abbreHation, to call a 
Light-metre. 
The chemical action of the solar rays is best expressed in these light-metres. The 
intensity of the chemical illuminating power which a unit of the earth’s surface receives 
either directly from the sun, from the difiuse light of a cloudless sky, or from clouds, 
can in this way be represented by the height of a column of gas, which, as we shall see 
in the sequel, when the atmosphere is clear, rapidly increases as the sun rises above the 
horizon until it reaches the meridian, as rapidly diminishing when the sim has passed 
that point ; and rising and falling under the influence of passmg clouds, forms, as it 
were, a wave accompanying the clouds in their motions, now ascending to a great elevation 
when bright white clouds reflect much light, and now sinking to a lower and permanent 
level when the sky is covered with grey clouds or obscured by mist. The mean daily, 
monthly, or annual height of this column of gas, dependent upon the latitude and 
longitude, regulates the chemical climate of a place, and points the way to relations for 
the chemical action of the solar rays, which in the thermic actions are already repre- 
sented by Isothermals, Isotherals, Isocheimals, and Isonomals. 
