424 M. Berthelot on a New Thermometer for measuring 



equally applicable, and without limit, to the measurement of very 

 low temperatures, even below the freezing-point of mercury. 

 We might construct a thermometer on the same principle capable 

 of use up to 1200° or 1500° by employing a reservoir of porce- 

 lain instead of glass. 



I will describe only the thermometer with the glass reservoir 

 adapted for measurements up to 500°. In the first place I will 

 give a description of the apparatus, indicate the method of its 

 graduation, discuss the theory of it, and, lastly, specify some of 

 the applications which I have already made of it. 



I. Description of the Thermometer. 

 The new thermometer (Plate VI. fig. 1) consists of an air-reser- 

 voir, a capillary stem, a reservoir filled with mercury, a graduated 

 scale, and a firm support for the whole. 



(1) The air-reservoir (B) is made of hard glass, of cylindrical 

 shape, 40 millims. in length and 12 millims. in diameter. Its 

 walls are moderately thin, and its internal capacity is about 4 

 cubic centims. These dimensions have been adopted in order to 

 allow of the introduction of the thermometer into a tubulated 

 retort of 125 cubic centims. capacity, but they may be enlarged 

 at will if larger vessels are to be used. 



The neck of the reservoir is fused to a capillary stem. 



(2) The stem tt hhllll mm nn is nearly 1200 millims. in 

 length, and its internal diameter is about one-fifth of a millimetre. 

 If reservoirs of greater ca~^city are employed, the bore of the 

 capillary tube may be augmented to half a millimetre, or even 

 beyond this. 



In every case the tube ought to be carefully calibrated by 

 means of a small column of mercury, in order to see if its bore 

 be perfectly uniform. 



The tube thus verified is fused to the neck of the reservoir B. 

 It has the form and disposition represented in fig. 1 ; the ver- 

 tical portion is about 730 millims. in length, and is terminated 

 by a large glass bulb, Q, with a cylindrical opening at the upper 

 end. The stem tt hh 11 11 mm nn ought to be of a single 

 piece and without any intermediate joining. This is an essential 

 point, since every joining produces a cavity, which interferes 

 greatly with the uniform capillarity. 



Another necessary precaution is to thoroughly dry both the 

 capillary tube and the air-reservoir B. This is done immediately 

 after sealing the stem and air-reservoir together, and when the 

 lower end of the latter is still open and drawn out to a point. This 

 open point is placed in communication with an aspirator through 

 which air is continually drawn, whilst the bulb Q and the capil- 

 lary tube are strongly heated, by means of a lamp, throughout 



