574 l'HILOSOPHXCAL TRANSACTIONS. [ANNO 1784. 



and more important motive for the use of charcoal was, that in consequence of 

 the remarkable porosity which it produces in the clay, it would probably diminish 

 the expansibility, by occasioning the mass to contain, under an equal surface a 

 much less quantity of solid or expansible matter. It may be objected to this 

 idea, that the expansions of metals, in Mr. Ellicott's* and Mr. Smeaton's-j- ex- 

 periments, do not appear to have any connection at all with their densities : but 

 the cases are by no means parallel ; for there the comparison lies between differ- 

 ent species of matter ; but here, between one and the same matter in different 

 states of compactness. If a metal could be treated as clay is in this instance, 

 that is, if a large bulk of any foreign matter could be blended with it, and this 

 matter afterwards burnt out, so as to leave the metallic particles at the same dis- 

 tances to which they had been separated by the mixture of it, we may presume 

 that the metal thus enlarged would not expand so much as an equal volume of 

 the solid metal. Such at least were the ideas which determined my choice to a 

 composition of clay and charcoal powder ; and being afterwards desirous of satis- 

 fying myself whether they had any foundation in fact, I have, since the experi- 

 ments were made, prepared some pieces of clay with and without charcoal, and 

 having burnt them in the same fire, I ground them at the sides, to make them 

 both fit exactly to the same division near the narrow end of the gage ; then, 

 examining their expansions by equal heats, I found the piece with charcoal to ex- 

 pand only -l part so much as that without ; and thus was fully satisfied with the 

 composition of the gage. 



To ascertain a fixed point on the scale for the divisions to be counted from, 

 the silver piece and gage were laid together for some time in spring water, of 

 the temperature of 50° of Fahrenheit : the point which the piece went to in 

 this cold state is that marked near the narrow end of the gage. The adjust- 

 ment is re-examined at the beginning and end of every succeeding experiment, 

 lest the repeated attrition, in sliding the pieces backwards and forwards, should 

 wear off so much from the surface of this soft metal as to occasion an error in 

 the minute quantities here measured. The apparatus is then exposed successively 

 to different degrees of heat, with the piece lying always in a part of the canal at 

 least as wide as it is expected to fill when expanded, otherwise the sides of the 

 gage would be burst asunder by its expansion, as I experienced in some of my 

 first trials. When the whole has received any particular degree of heat desired, 

 the piece is cautiously and equably pushed along, till it is stopped by the conver- 

 gency of the sides, of which I always find notice given by the gage itself (which 

 is small and light) beginning to move on the continuance of the impulse. A 

 flat slip of iron, a little narrower than the piece, bent down to a right angle at 



* Phil. Trans, vol. 47, p. 4S5. f Ibid. vol. 4S, p. 6l2. 



