[ 3 l8 1

4. There is one cafe however, in which a high polishmay, on the lame principle, produce blackness, in bodiesotherwise even white. We find that specula of white me-tal or of quicksilvered glass, which reflect the rays of lightto one point or in one direction, look always dark, unlesswhen the eye is directly opposed to the reflected rays.

5. As the absorption of the luminous rays, except inthe cafe just mentioned, makes the physical cause of black-ness ; it is concluded that black bodies receive heat morefreely than others. Black marble or tiles, exposed to thesun, become sensibly hotter than white ones. Black pa-per is kindled by a burning-glass much sooner than white,and the difference is strongly marked : a burning-glass,too weak to have any visible effect at all upon white paper,shall readily kindle the fame paprr rubbed over with ink.Hence black clothes, when wetted, are said to dry faster;black habits, and rooms hung with black, to be warmer;black mould to be a hotter foil for vegetables; and gar-den walls, painted black, to answer better for the ripeningof wall fruit; than those of lighter colours.

6. It is not however to be affirmed that the like dif-ferences obtain in the impressions made by common sire.Black paper, held to the fire, does not seem to be affectedsooner, or in a greater degree, than such as is white. Itmay be proper to observe also, that the combustibility ofthe paper may be increased, by impregnating it with sub-stances of themselves not combustible, and which give nocolour to it. This is the foundation of one of the. sympa-thetic inks, as they are called, made of a strong solutionof sal ammoniac in water, which, though colourless whenwritten with on paper, becomes very legible on exposingthe paper to the fire ; that is, it occasions the parts moist-ened with it to scorch or burn, before the rest of the paperis hurt, to a brown or black. All the salts I have tried

produced