368 PEOrESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 
sensibility of the gaseous mixture is determined by obsening the action produced by a 
gas-flame of constant dimensions. 
As soon as a maximum action has been reached, which generally occurs in from three 
to six, sometimes even from eight to nine days, according to cu'cumstances, the instrument 
can be used for comparative measurements extending over a period of several months, 
and requires only a short preparation to enable it to give correct results throughout the 
day. All that is necessary for this purpose, is to pass the gas evolved from fresh hydro- 
chloric acid, for an hour through the manometer m, and afterwards the glass stopcock 
/a is opened, and the gas allowed to pass through the apparatus for from fifteen minutes 
to an hour, after which the maximum action has generally been attained. As soon as 
the stopcock is shut, in order that the observations may be begun, the balance-wire of 
the gyrotrope is reversed, so that the hydrochloric acid is constantly decomposed by a 
very feeble current. If this precaution be neglected, the equilibrium of the gases dis- 
solved in the acid is soon destroyed, owing to a contact-combination of the chlorine and 
hydrogen taking place on the surfaces of the carbon-poles. Above all, access of air in 
the washing-bulbs and in the insolation-vessel must be most carefully avoided. If the 
insolation-vessel be removed from the apparatus for merely a few moments, a satiuntion 
lasting from two to six days is necessary before the maximum degree of sensibility is 
again reached. The amount of foreign gas which is sufficient to render the indications 
of the instrument incorrect, cannot approach to a billionth part of the total volume of 
gas. We shall have occasion to return to this remarkable fact in a later part of the 
research ; but we may here remark, that when the maximum action has been reached, 
the explosive nature of the gas is so great, that a thin glass bulb of the size of a pigeon’s 
egg filled with the gas, immediately explodes on exposure to the diffuse hght of an over- 
clouded sky at an open window. On taking the apparatus to pieces, portions of the gas 
have several times exploded on exposure to evening light, even after the sun had disap- 
peared below the horizon. 
Having thus determined the conditions which must be fulfilled in order to obtain 
comparative results, we proceed to the examination of the influence Avhich the heat 
radiated from the source of light, and the heat of combustion CA'olved by the combina- 
tion of the gases, exert upon the indications of the instrument. 
The relation between the volumes of the insolation-vessel used in these experiments, 
and the volume occupied by one division on the obseiwation-tube, Avas as 7430 to I. 
Hence, if this A-essel were heated from 0° C. to 0°‘0366 C., the AA'hole mass of gas would 
be increased by a volume represented by one dmsion of the tube. Our instrument is 
therefore not only a photometer, but also a A^ery sensitAe air-thermometer, and all experi- 
ments Avith it must be conducted in a space AAuthin AAliich the changes of temperature 
are not large enough to cause any perceptible error. 
This condition Avas fulfilled by experimenting in a room of AA'hich the outer AA'all aaus 
at no time of day exposed to the direct sunlight, and the only AvindoAv completely closed 
from without. 
