298 



Mr. T. Tate on the Construction of 



have just the form of those of ordinary alum ; they are combina- 

 tions of the octahedron and cube. They have all the lustre of 

 alum crystals, than which they are somewhat more soluble. 

 When heated, they give off water and finally selenious acid and 

 oxygen. 



XXXVII. On the Construction of a new Air-thermometer. 

 By Thomas Tate, Esq.* 



AIR, from the uniformity and extreme range of its expansion 

 under an increase of temperature, seems to be the sub- 

 stance best fitted for measuring changes of temperature. But 

 the indications of the ordinary air- thermometer at present in use 

 are subject to derangement from the varying pressure of the 

 atmosphere. In the differential thermometer, it is true, the 

 atmospheric pressure is altogether excluded ; but then this in- 

 strument cannot be employed, as in the case of the common 

 thermometer, for indicating the temperature of the surrounding 

 air. Now in the instrument here proposed (represented in the 

 annexed diagram), the variations of pressure of the atmosphere 

 are corrected by an easy adjustment ; and the air in the instru- 

 ment being thus maintained at a constant pressure, the volume 

 of this air becomes a true measure of the change of temperature. 

 G and N are small glass globes, about an inch and a tenth in 

 diameter, connected with the bent 

 tubes, as shown in the diagram; 

 A K C a barometer-tube about 

 20 inches in length; E L a wide 

 thermometer-tube about T J T tn °f 

 an inch diameter ; LNM a tube, 

 somewhat larger in diameter, con- 

 nected with the globe N, which ter- 

 minates with an open tube; B an 

 india-rubber ball, about 1 £ inch dia- 

 meter, having an open tube proceed- 

 ing from it, forming an air-tight 

 connexion with the globe N ; S the 

 head of an adjustment-screw, which, 

 working in a hollow screw formed in 

 the cross bar H I, compresses the 

 ball B as may be required ; A B C and 

 M Ij D are mercury columns ; the 

 globe G and tube A E D contain dry 

 air, and the space above the mercury 

 C is a Torricellian vacuum, so that the 



* Communicated by the Author. 



