4 
PEOFESSOR TYNDALL ON THE ABSORPTION AND 
inches in diameter, the ends of which were furnished with brass appendages to receive 
the plates of rock-salt. Each plate was pressed firmly against a flange by means of a 
bayonet joint, being separated from the flange by a suitable washer. Various descrip- 
tions of leather washers were tried for this purpose and rejected. The substance finally 
chosen was vulcanized india-rubber very lightly smeared with a mixture of bees-wax and 
spermaceti. A T-piece was attached to the tube, communicating on one side with a 
good air-pump, and on the other with the external air, or with a vessel containing the 
proper gas. 
The tube being mounted horizontally, a Leslie’s cube containing hot water was placed 
close to one of its ends, while im excellent thermo-electric pile, connected with its 
galvanometer, was presented to the other. The tube being exhausted, the calorific 
rays sent through it fell upon the pile, a permanent deflection of 30° being the con- 
sequence. The temperature of the water was in the first instance purposely so arranged 
as to produce this deflection. 
Dry air Avas noAV admitted into the tube, while the needle of the galvanometer was 
observed Avith all possible care. Even by the aid of a magnifying lens I could not 
detect the slightest change of position. Oxygen, hydrogen, and nitrogen, subjected to 
the same test, gave the same negative result. The temperature of the Avater Avas sub- 
sequently lowered so as to produce a deflection of 20° and 10° in succession, and then 
heightened till the deflection amounted to 40°, 50°, 60° and 70°; but in no case did the 
admission of air, or any of the above gases into the exhausted tube, produce any sensible 
change in the position of the needle. 
It is a well-knoAvn peculiarity of the galvanometer, that its higher and loAA'er degrees 
represent different amounts of calorific action. In my instrument, for example, the 
quantity of heat necessary to move the needle from 60° to 61° is about tAventy times that 
required to move it from 11° to 12°. Noav in the case of the small deflections aboA’e 
referred to, the needle Avas, it is true, in a sensitLe position ; but then the total amount 
of heat passing through the tube Avas so inconsiderable that a small per-centage of it, 
CA’en if absorbed, might Avell escape detection. In the case of the large deflections, on 
the other hand, though the total amount of heat Avas large, and though the quantity 
absorbed might be proportionate, the needle Avas in such a position as to require a 
A*ery considerable abstraction of heat to produce any sensible change in its position. 
Hence arose the thought of operating, if possible, Avith large quantities of heat, AA^hile 
the needle intended to reveal its absorption should continue to occupy its position of 
maximum delicacy. 
The first attempt at sohing this problem was as folloAvs : my gahunometer is a differen- 
tial one ; the coil being composed of tAVO wires Avound side by side, so that a current could 
be sent through either of them independent of the other. The thermo-electric pile AA-as 
placed at one end of the tin tube, and the ends of one of the galvanometer AV'ires con- 
nected with it. A copper ball heated to low redness being placed at the other end of 
the tube, the needle of the galvanometer was propelled to its stops near 90°. The ends 
