C 575 I 
XXXVIII. Experiments on Ignited Sub fiances* By Mr. 
John Whitehurft, in a Letter to James Stuart* 
Ef quire, F. R. S. 
TO JAMES STUART, ESQ. 
London r 
SIR, Nov. 29, 1775. 
R. May 23, '"T^HE experiments of Mr. buffon upon 
A ignited bodies feem to prove, that, when 
heated to the degree he mentions, they are more pon- 
derous than when cold. The experiments which I have 
made on heated metals, fuggeft a different idea, and con- 
tradict the fa£t he relates ; fo that I am induced to be- 
lieve, that fome circumftance, not attended to, has intro- 
duced a miftake in the relation this learned philofopher 
has publifhed as the refult of his inquiry. 
His experiment hands thus recorded (Suppl. Nat. Hilt, 
vol. II. p. 11.) : a mafs of iron, after receiving a white 
heat, weighed 49 lbs. 9 oz. ; when reftored again to the 
temperature of the atmofphere 49 lbs. 7 oz. Hence he 
concluded, that the igneous particles, contained in the 
heated iron, increafed its abfolute weight 2 ounces. 
2 My 
