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plates which compofe their furfaces, of whatever den- 
fity thofe plates be (and which may be of fuch ad- 
mirable ufe to explain the colours, and perhaps, in due 
time, the confiituent parts and internal firu&ure of 
natural bodies) I have been fo happy as to hit upon 
a method of ill nitrating and confirming, by means of 
electrical explofions. Thefe, being received upon 
the furfaces of all the metals, change the colour of 
them, to a confiderable difiance round the fpot on 
which they are difcharged, fo that the whole Ipace is 
divided into a number of concentric circular fpaces, 
each exhibiting all the prifmatic colours ; and perhaps 
as vivid as they can be made in any method what- 
ever. ' 
It was not by any reafoning a priori, but by a mere 
accident, that I firfi difcovered thefe colours. Having 
occafion to take a great number of explofions, in order 
to afcertain the lateral force of them ; I obferved that 
a plate of brafs, on which they were received, was 
not only melted, and marked with a circle, by a 
fufion round the central fpot, but likewife tinged, 
beyond this circular fpot, with a green colour, which 
I could not eafily wipe out with my finger. Struck 
with this new appearance, I replaced the apparatus, 
and continued the explofions; till, by degrees, I per- 
ceived a circle of red beyond the fainter colours; and, 
examining the whole with a microfcope, I plainly 
difiinguilhed all the prifmatic colours, in the order of 
the rainbow. The diameter of the red, in this in- 
fiance, happened to be one third of an inch, and the 
diameter of the purple about one fourth. 
Pleafed with this experiment, I afterwards purfued 
and diverfified it in a great variety of ways, the refult of 
which I fhall comprife in the following obfervations. 
i. When 
