Specific Gravity of Vapours and Gases. 9 



axis of 8 centimetres, and a minor axis of 5*5 centimetres. 

 From this cylinder, at seven places equally distant from each 

 other, project copper wires 7 to 8 millimetres thick, which are 

 riveted and brazed into the copper cylinder. 



The wires pass through the lamp-chimneys in such a manner 

 that they lie at right angles to the vertical axes of the cones of 

 the non-luminous flames, and are placed at the point where the 

 temperature is highest and most uniform. By this arrangement 

 it is seen that the temperature of the interior of the copper vessel 

 is rendered very nearly independent of the height of the flame, 

 and is almost entirely regulated by the distance of the lamps 

 from the cylinder. To avoid the irregularities in the distribution 

 of heat conducted in this way, a somewhat smaller copper vessel 

 of the same shape, also closed below, is firmly connected to the 

 outer one, in such a manner that a thin layer of air separates the 

 one from the other. 



The inequalities in the heating of the outer copper jacket are 

 so completely dispelled by radiation from this layer of air, that, 

 when the lamps are properly placed, the interior of the inner 

 copper jacket retains a constant temperature for a very consider- 

 able time, as soon as an equilibrium is established between the 

 heat conducted by the wires and that which passes off by the 

 air. The upper ends of both copper vessels can be closed with 

 covers pierced with three corresponding holes, through two of 

 which project the points of the glass vessels which are being- 

 heated, and through the third a thermometer. In order to ob- 

 tain as constant a temperature as possible, the apparatus must 

 be carefully screened from draughts of air and other cooling or 

 heating influences, and the heights of the flames kept nearly 

 equal by means of a gas stopcock or by a regulator. If the 

 pressure of gas is only equal to a water-column of j of an inch 

 in height (the pressure which the gas-establishments usually 

 pledge themselves to maintain), the lead pipe which leads the 

 gas to the chief tube of the apparatus must be scarcely less than 

 an inch in diameter, so that the flames may reach a height suffi- 

 cient to keep both the copper rods in the middle part of the 

 flame, and not to have the upper rod heated only by the extreme 

 point of the flame. The flames can be arranged in three different 

 positions equidistant from the copper vessel. These are recog- 

 nized by notches on the conducting-rods. The three positions 

 represent the temperatures 123°*6, 144 0, 6, and 176° C. If the 

 conducting-rods be surrounded by a second row of flames, the 

 temperature rises to 210° ; and if a greater number of conducting- 

 rods are employed, it is easy to obtain constant temperatures 

 above 300° C* 



* I use constantly, for chemical operations which require a constant 



