VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 573 



is a piece of fine silver : this is to be gently pushed or slid along, towards the end 

 fh, till it is stopped by the converging sides of the canal, k is a little vessel 

 formed in the gage for this particular series of experiments, the use of which 

 will appear hereafter. 



The contraction, which the thermometer pieces receive from fire, is a perma- 

 nent effect, not variable by an abatement of the heat, and which accordingly -is 

 measured commodiously and at leisure, when the pieces are become cold. But 

 the expansion of bodies is only temporary, continuing no longer than the heat 

 does that produced it ; and therefore its quantity, at any particular degree of 

 heat, must be measured in the moment while that heat subsists. And further, 

 if the heated piece was applied to the cold gage, the piece would be deprived of 

 a part of its heat on the first contact ; and as the gage receives some degree of 

 expansion from heat as well as the piece, it is plain that in this case the piece 

 would be diminished in its bulk, and the gage enlarged, before the measurement 

 could be taken. It is therefore necessary that both of them be heated to an 

 exact equality ; and in that state we can measure, not indeed the true expansion 

 of either, but the excess of the expansion of one above that of the other, 

 which is sufficient for the present purpose, as we want only a uniform and 

 graduated effect of fire, and it is totally immaterial whether that effect be the 

 absolute expansion of one or the other body, or the difference of the two, pro- 

 vided only that its quantity be sufficient to admit of nice measurement. 



Some difficulties occurred with respect to the choice of a proper matter for 

 the gage ; the essential requisites' of which are, to have but little expansibility, 

 and to bear the necessary fires without injury. All the metals, except gold and 

 silver, would calcine in the fire : those two are indeed free from that objection, 

 and accordingly it is of the most expansible of them that the piece is made ; but 

 if the gage also was made of the same, the measure itself would expand just as 

 much as the body to be measured, and no expansion at all would be sensible; 

 and though the gage was made of one of those metals, and the piece of the 

 other, the difference between their expansions would be too small to give any 

 satisfactory results, as more than ■§■ of the real expansion of either would be lost 

 or taken off* by the other. 



For these reasons I had recourse to earthy compositions, which expand by 

 heat much less than metallic bodies, and bear the necessary degrees of fire with- 

 out the least injury. I made choice of tobacco-pipe clay, mixed with charcoal in 

 fine powder, in the proportion of 3 parts of the charcoal to 5 of the clay by 

 weight. By a free access of air, in the burning by which the gage is prepared 

 for use, the charcoal is consumed, and leaves the clay extremely light and 

 porous ; from which circumstance it bears sudden alternations of cold and heat, 

 often requisite in these operations, much better than the clay alone. Another 



