transparency of water also favors this view; for the most transparent crystal when broken and ground and reduced to powder loses its transparency; the finer the grinding the greater the loss; but in the case of water where the attrition is of the highest degree we have extreme transparency. Gold and silver when pulverized with acids [acque forti] more finely than is possible with any file still remain powders,[1] and do not become fluids until the finest particles [gl' indivisibili] of fire or of the rays of the sun dissolve them, as I think, into their ultimate, indivisible, and infinitely small components.
Sagr. This phenomenon of light which you mention is one which I have many times remarked with astonishment. I have, for instance, seen lead melted instantly by means of a concave mirror only three hands [palmi] in diameter. Hence I think that if the mirror were very large, well-polished and of a parabolic figure, it would just as readily and quickly melt any other metal, seeing that the small mirror, which was not well polished and had only a spherical shape, was able so energetically to melt lead and burn every combustible substance. Such effects as these render credible to me the marvels accomplished by the mirrors of Archimedes.
Salv. Speaking of the effects produced by the mirrors of Archimedes, it was his own books (which I had already read and studied with infinite astonishment) that rendered credible to me all the miracles described by various writers. And if any doubt had remained the book which Father Buenaventura Cavalieri[2]
[87]
has recently published on the subject of the burning glass [specchio ustorio] and which I have read with admiration would have removed the last difficulty.
Sagr. I also have seen this treatise and have read it with
- ↑ It is not clear what Galileo here means by saying that gold and silver when treated with acids still remain powders. [Trans.]
- ↑ One of the most active investigators among Galileo's contemporaries; born at Milan 1598; died at Bologna 1647; a Jesuit father, first to introduce the use of logarithms into Italy and first to derive the expression for the focal length of a lens having unequal radii of curvature. His "method of indivisibles" is to be reckoned as a precursor of the infinitesimal calculus. [Trans.]
