282 PHILOSOPHICAL TRANSACTIONS. [ANNO 1762. 



plied with mercury for the common thermometers from the Spanish or Hunga- 

 rian mines. 



2. We ought to be assured also, that all the clay made use of for these ther- 

 mometers, is perfectly similar. For this purpose, it will be best to dig it out of 

 the earth in considerable quantity at once, an extent of some square feet or yards 

 in area, and to the depth of 6 or 7 yards or more from the surface, and to mix. 

 the whole thoroughly together, previous to the further preparation already men- 

 tioned. When the first quantity is exhausted, another perpendicular column 

 may be dug from the same bed, close to the first, to the same depth, and pre- 

 pared in the same manner ; by which means we may be assured of its similarity 

 with the former parcel, and that it will diminish equally in the fire. 



3. This clay, dried by the summer heat, or in a moderately warm room, or 

 with more heat before a fire, has not been observed to differ in degree of dryness. 

 After being so dried, it loses about a 10th part of its weight in the heat of boil- 

 ing water, about as much more in that of melted lead, and from thence to a red 

 heat J O parts, in all -£fa. Each of these heats soon expels from the clay its de- 

 terminate quantity of matter, chiefly air; after which, the same heat, though 

 continued for many hours, has no further effect. I had some hopes, that the 

 graduation of the common thermometer might be continued, on this principle, 

 up to the red heat at which the shrinking of the clay commences, so as to 

 connect the 2 thermometers together by one series of numbers ; but the loss of 

 weight appears not to be sufficiently uniform, or proportional to the degree of 

 heat, to answer that purpose ; for it was found to go on quicker, and bladders 

 tied to the mouths of the vessels in which the pieces were heated, became more 

 rapidly distended, at the commencement of redness than at any other time. 

 From low red heat to a strong one, such as copper melts in, the loss of weight 

 was only about 2 parts in 100 ; though the difference between these 2 heats ap- 

 pears to be much greater than what the same loss corresponds to in the lower 

 stages. After this period, the decrease of weight entirely ceased. The vapours 

 expelled from the clay, caught separately in the different degrees of heat, seemed, 

 from the few trials made with them, to consist of common air mixed with fixed 

 air. They all precipitated lime-water ; that which was first extricated, exceeding 

 weakly ; the others more and more considerably ; but the last not near so 

 strongly as the air expelled from lime-stone in burning. None of them were 

 inflammable. 



The thermometric pieces may be formed much more expeditiously than in the 

 single mould, by means of an instrument used for similar purposes by the potters. 

 It consists of a cylindrical iron vessel, with holes in the bottom, of the form and 

 dimensions required. The soft clay, put in the vessel, is forced by a press down 

 through these apertures, in long rods, which may be cut while moist, or broken 



