«m6 J Dr. Fordyce’s Account of 
•J 
favoured, as in former experiments the refults had been in favour 
of the iron apparatus acquiring the greateft heat. Every thing, 
therefore, was the fame, except that the iron and pafteboard 
were of different weights, of different capacities of heat, and 
of different degrees of readinefs to acquire heat, and commu- 
nicate it. 
It is evident, that a greater quantity of heat was a&ually 
produced in the apparatus with the iron diaphragm : for al* 1 
though in the firft two or three minutes the pafteboard became 
hotter than the iron, yet as foon as the iron began to be fenli- 
bly heated, it became hot fafter than the pafteboard, and 
adtually became hotter, and even continued to do fo, when the 
pafteboard no longer could produce more heat than was difli- 
pated from the furface of the apparatus into the air. When 
they were fet in an air equally cold the apparatus with the iron 
diaphragm was longer in cooling, although they were both of 
ithe fame degree of heat when fet by. 
This greater quantity of heat I afcribe to the iron’s taking 
r the heat from the black paint fafter than the pafteboard, as 
being a better condu&or. Juft as if a plate of glafs was placed 
on a plate of fteel, and another, perfedly fimilar, was placed 
on a plate of clay, and both were placed equally among equal 
vibrating bodies. In this cafe it is clear, that much greater 
vibration would take place if the fame means of exciting it 
were applied to that plate of glafs attached to the plate of fteel 
than if they were applied to that attached to the clay. I do 
not mean to fay, that heat is vibration ; but merely to illuftrate 
my idea of heat’s being only a quality, and not a fubftance. I 
am led to this not only by this experiment now related, but by 
various other confiderations, which I fhall not now infift upon, 
as they are not fufficiently finilhed to be laid before this Society. 
I fhall 
