88 Remarks on Elasticity. 
This is eminently the case with air. A correlative property of elasticity, is of 
course compressibility- 
Air will bear considerable compression ; in the state in which we breathe it, it is 
pressed by a column of mercury 30 inches in length. 
Take a tube closed at one end, and at the other furnished with an air-tight 
piston. Suppose the piston loaded with a weight = to 30 inches of mercury. Then 
as there is also the weight of the atmosphere, the air is pressed by a double weight, 
and will be forced into one half the original space. Now supposing the tempera- 
ture constant, the piston will rest say at 4, ah being equal to be. Now heat the 
cylinder, the pistion will rise to a. The weight then which a given bulk of air 
will endure without compression depends on the temperature. 
The relation which the temperature bears to the weight is as follows : Under 
a pressure of 30 inches of mercury, air expands from 1 to 1 375 for 1 80° of Fahren- 
heit’s thermometer, being 1 at 32 and 1. 375 at 212. At 212° then, it will bear 
42.2 inches of mercury, without being compressed! at 32°, 30 inches; at 148° , (i e. 
180° below 32°) it will bear 18,75. At 328° — ,7-5 only; and at 448° — 0. That 
Is to say, that if it be cooled down to 448 — , air will not bear compression, so as 
to restore itself ; will not support a weight ; most probably it would liquefy. 
Vi'hat is then the cause of this curious property of heat, by which air may be 
made to support a greater weight ? 
We sec that a weight condenses air into less space, by which it is enabled to sop- 
port that weight without further Condensation. In this case the particles are 
brought nearer to each other ; a greater number are contained in a given space ; that 
is, the specific gravity is increased. Now heat the air, and it will expand and 
force up the piston or weight, till the spriug of the air be a precise balance to the 
pressure. We see then, that heat may neutralize the power of gravity acting on an 
elastic fluid ; in other words, that an elastic fluid heated , may ■produce the same ef- 
fects as gravity. Another conclusion we may draw is this, that the rarity of the fluid 
is no obstacle to its action. Take the instance before, suppose oe = l ft and 
the piston pressing with a force of 60 inches of mercury. Heat it to 212°, and it will 
expand to 1 375 ft. and still support 60 inches. Tile specific gravity is here only 
■ft of what it was. Heat it again to 392, it will expand to 1 -75 still the same pres- 
sure. Heat it to 572, it has doubled in volume and consequently is only t the weight 
or density it was bo that by 540° of heat one half the quantity ol' matter is made 
to support the same weight Again if we take hydrogen, which is 5*5 of the 
specific gravity of air ; the same conclusions will hold. 
May we then proceed a little further and say — As gravity may be counteracted 
by elastic force ^derived from heat ; as the latter may in every case be substituted 
for the former ; is it not probable that gravity is the effect of some elastic medium of 
extreme tenuity, of such tenuity as will readily enable it to penetrate through every 
substance ? The reduction of two classes of phenomena to one cause, is a strong 
temptation to adopt such an opinion. May it not be owing to the action or elastic 
force of this fluid that all bodies gravitate or are pressed downwards ; nay, even that 
the elastic force of a gas is owing ? and this latter conclusion is greatly strengthened 
by the known fact, that all gases and even vapours expand after the same law. 
In the state of gas, the particles of the body are so widely separated as to afford 
room for this agent to exert itself. Thus, steam has in the same space only Ia L_ 0 f 
the number of the particles which water has. 
But further : what if this elastic medium which we may suppose to fill all space, be 
"the cause, also of heat ? Thus we should account for equilibrium of temperature, 
commonly explained on the principle of radiation. Such an elastic substance if it 
be also capable of entering solid bodies and combining with them as we see gases 
do, would account for what is called latent heat. There are, however, I think, 
some difficulties in this latter hypothesis difficult to be removed, in particular the 
phenomena of combustion, &c. 
But again if we suppose the phenomena of heat to be something different from 
the mere motion of this elastic medium, we shall be then forced to admit, that tem- 
perature will modify the force of gravity : such a conclusion appears pretty well 
established by Mr. Herapath, who has ingeniously supported the above opinion. 
Tyro. 
