AND STATICS UNDER THE INFLUENCE OF LIGHT. 
363 
time quite freely suspended in the box, the fine suspension wires on the one hand 
allowing free movement to the galvanometer wherever and whenever it may be 
required, and, on the other hand, exerting no directing influence whatever on its mass. 
The small vibrations of the walls of the room were thus allowed to affect the 
copper wire at the ends, and were weakened almost to zero before they w r ere 
transmitted to the middle part of the wire. These vibrations in the middle part 
of the wire were further weakened by the two rubber rings upon which the box 
with the galvanometer was suspended, and, lastly, the galvanometer itself was 
made independent both of the vibrations of the box and of the leads. In this way 
excellent results were obtained, the movements of the spot of light when the scale 
was removed from the galvanometer 1 or 1’2 metres not exceeding OT millim. 
Having now, from the acetylene flame, a deflection of the spot of light of about 
10 to 15 centims., each reading was brought (and with it the possibility of adjustment 
of the intensity of light to the right or left) to an accuracy of about O'l per cent., 
an accuracy far greater than that required for the research. 
Determination of the Value of the Observed, Deflection of the Galvanometer or of the 
Intensity of Light in Standard Units. (Fig. 7.) 
With the arrangement described above, the deflection of the spot of light depends 
upon conditions which may easily vary according to the circumstance and time. 
Assuming that the distance of the source of light from the thermopile is fixed, that 
the flame is in the correct position, that the distance of the scale from the mirror is 
fixed, still, if the india-rubber rings should become a little stretched in time, or 
any similar accident happen, the sensitiveness of the galvanometer would vary. It 
is, therefore, necessary that measurements of the intensity of light should be made 
independent of variations in the sensitiveness of the galvanometer. It is further 
desirable to be able to express at once the intensity of light in standard units 
independently of any given arrangements of the photometer, &c. For this reason, 
directly after the measurement of the light by means of the thermopile was made, 
a second measurement was made with a Clark cell and manganin resistances, as 
given in the above diagram, thus determining the value of the deflection, as caused 
by the light, in units given by the Clark and manganin resistances. 
The Remaining Parts of the Apparatus, &c. 
The Bath. (Fig. 5, p. 352.) 
The reaction vessel with the mixture of chlorine and hydrogen was immersed in 
a water-bath and there exposed to light. The bath contained quartz windows, and 
the reaction vessel was placed behind one of them, the manometer M (E in fig. 1) 
remaning outside the bath. The volume of the gas in the capillary tubes and in the 
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