Notes. 419 



New Hot Stage. 

 By W. S. Lazarus-Barlow, M.P., F.E.C.P. 



Plate VII. 



The inventor exhibited and described at the June Meeting a new 

 form of warm stage, which can be heated by either gas or oil. 

 The principle of the apparatus is that of a balance and a mano- 

 meter combined. The stage itself is a brass box, which contains 

 a series of flattened and communicating glass bulbs, connected 

 with a mercury manometer of particular shape. A glass tap is 

 fused into the manometer between it and the stage itself. Over 

 the mercury in the open limb of the manometer is an iron float, 

 suspended by silk from one arm of the beam of a balance. This 

 beam is supported on a knife-edge, and is provided with an ad- 

 justable weight at the end distal from the warm stage, and a silver 

 rod suspended by loops of platinum-iridium at the proximal end. 

 The silver rod is bent downwards at one end, and is placed at 

 right angles with the beam, both being in the horizontal plane. 

 The bent portion of the silver rod dips into a small bath, which 

 is brazed to the side of the warm stage, and contains paraffin of 

 M.P. about 58°. 



The apparatus works as follows. Heat from a flame is applied 

 to the silver rod at the unbent end, and is conducted to the 

 paraffin in the bath at the side of the stage, and thence to the stage 

 itself. Variations in the temperature of the stage are conveyed to 

 the air in the glass bulbs within the stage, and express themselves 

 by expansion or contraction of that air, and therefore by varia- 

 tions in the level of the mercury in the manometer. These 

 variations of the level of the mercury allow the entire weight of 

 the iron float in the distal limb of the manometer to act upon the 

 beam (when the mercury recedes sufficiently to lose contact with 

 the float), or remove the entire weight of the float from the beam 

 (when the mercury rises sufficiently to slacken the silk thread 

 connecting the beam and the float). Intermediate positions of 

 the mercury, of course, allow intermediate proportions of the 

 weight of the float to act upon the beam. Hence the weight on 

 the side of the beam towards the warm stage varies inversely as 

 the volume of the air within the glass bulbs, i.e. inversely as the 

 temperature of the stage itself. Consequently (the beam being 

 free to move about its fulcrum) the cooler the stage the deeper 

 the heated silver rod is plunged into the bath of paraffin, and 

 vice versa ; this greater immersion of the heated silver rod heats 

 the stage, expands the air in the bulbs, raises the mercury in 

 the distal limb of the manometer, supports the iron float, and 

 allows the beam to revert to its original horizontal position — or 



2 F 2 



