246 Prof. M. C. Potter. Bacteria as [Jan. 18, 
A solid block of paraffin formed the basis of the key. Into this block 
four holes were bored to serve as mercury cups and these were connected 
in pairs by a metal bridge. One cup of each pair received the wires. 
from the galvanometer and into the others were fixed short glass tubes. 
Through these tubes the ends of the wires from the incubator could be 
raised or lowered to break or make a contact. The key was packed with 
dried cotton wool and enclosed in a box, the two glass tubes slightly projecting 
through perforations in the lid. 
Section of Special Key. 
A, Paraffin block with two mercury cups. 
B, Lead to galvanometer. 
C, Lead from incubator. 
D, Connecting bridge of copper wire. 
All the junctions of the different metals were thus carefully protected 
from light or changes of temperature which might set up an electric current, 
however small, and thus give rise to experimental error. | 
The accuracy of the measurements of temperature, as recorded by means. 
of the thermopile and galvanometer, is shown by the fact that the calculated. 
differences of temperature were found to correspond with those registered. 
by standard mercurial thermometers under the same conditions. 
The Hearson’s incubator was first regulated for 20° C., but as this. 
apparatus was heated from one side, it was necessary to ascertain whether 
the temperature inside was uniform. The terminals of the thermopile were 
packed with dry cotton wool in glass jars, and placed in different positions. 
inside the incubator, and by this delicate method a difference of temperature: 
of 0°09 C. was detected between the two sides. On discontinuing the heat. 
and allowing the incubator to assume the laboratory temperature, 14° C., 
this difference gradually disappeared, and I was able to determine, by 
