xui Historical Sketch of the Physical Sciences, 1818. 
mercury in the two legs was accurately measured at each temper- 
ature, and this difference indicated the specific gravity of the 
mercury in the hot leg of the syphon, or the expansion which 
it had sustained. The following table shows the dilatation of 
mercury for a degree centigrade at the various temperatures 
centigrade, indicated in the first column of the table, and mea- 
sured by an air thermometer. 
Temperature indicated by 
Temperature, Expansion of mercury. the dilatations of the 6 
: supposed uniform. 
0° HATA Se i ape eta al pis 0:00 
15s le ew eps ada A adage, Bp 100.00 
2h ke eg eagle Seta AIS 4 4 | 
S.uveageegieley eo petal La on mde a 
From these experiments we learn, that if we employ an air 
thermometer to measure the temperature, and if we suppose the 
expansion of air to be equable, or, in other words, that equal 
increments of heat occasion equal increments of bulk, then mer- 
cury is more expanded by heat at high temperatures than at low 
. temperatures, or its expansibility slowly increases as the temper- 
ature augments. This rate increases so slowly that between 
32° and 212° it sensibly corresponds with the expansion of air ; 
so that we may consider the expansion of mercury as equable 
up to the temperature of 212°. Between 212° and 392° there is 
a small increase in the expansibility. There is another small 
increase between 392° and 582°. The first of these increments, 
as may be seen by the third column of the preceding table, is 
equivalent to 4°61° of the centigrade scale. The second incre- 
ment is equivalent to 14°15° of the same scale. The consequence 
of this is, that 200° centigrade on the air thermometer is the 
same as 204°61° on the mercurial thermometer, and 300° on the 
air thermometer the same as 314°15° on the mercurial thermo- 
meter. 
As we haye no other method of measuring temperature but 
the expansion produced, it is obviously necessary in the first 
place to fix upon some body as a standard by supposing its 
expansion to be equable. Our authors have made choice of azr. 
They have taken it for granted that its expansion is equable, and 
have been induced in consequence to compare with it the expan- 
sions of all other substances. When we consider that it has 
been established by experiments which appear satisfactory, that 
all gases undergo the same change of bulk by the same incre- 
ments of heat between 32° and 212°; and when we consider 
further the peculiar constitution of these elastic fluids, it will, I 
think, be acknowleged, that they are at least as likely, if not 
more so, to expand equably when heated, as any substances in 
nature ; yet I think it a material defect in the paper of which I 
am at present giving an acount that a rigid examination of this 
