[ 459 ] 
exerted. And upon removing the preflure, the bladder 
will immediately recover its firft form. 
This yielding or giving way of the form, and then 
aftei wai ds i ecovering it, proves an elaftic fubftance ex- 
ifting within the bladder, and between the two lides 
where the preflure is employed. 
In like manner when two glafs prifms, or the ob- 
jedt glaffes of two long Tellelcopes prefs upon each 
othei with their own weight only, philofophers know, 
by the phenomena of light, that they do not touch : 
and that there mu ft be Something between the glafles 
lo keep them at a diftance. T. hey alfo know, by the 
like phenomena, that more prefting is required to 
bring them nearer to a contad 3 and that when the 
preflure is removed, they immediately recover their 
firft diftance. 
Now this yeilding or giving way, and the recovery 
of the diftance between the glafles, proves the ex- 
istence or fome elajlic fubftance between them ref- 
pedively. Since we find the effeds of applying, and 
removing, the preflure, exadly fimilar to the cafe with 
the bladder. 
Hence it is evident that the fame elaftic fubftance 
or medium caufes prifms, and convex glafles, when 
prefled againft each other, to exhibit feveral rings 
of different colours 3 by having its denfity vari- 
ed : and that it occafions all bodies to ad upon 
light at a diftance, by refleding, refrading, and 
infleding it 3 and light [to ad upon bodies, at a 
diftance, by caufing a motion of their parts, and 
heating them. 
This is the medium then which gives rife to the reftft - 
ance found inelefflric experiments. 
For 
