13 6

PHOTOMETER.

of a good wax candle varies from 10 O to about <50; if namelysuch a candle when newly snuffed, and burning with its greatestbrilliancy, emits a quantity of light = 100, its light will be re-duced to 20, and even as low as l6, when it is suffered to burnvery dim for want of being snuffed. The loss of light emittedby a tallow candle under similar circumstances, was found to bemuch greater. A particular account of the principles on whichCount Rumford’s photometer is founded, together with somemasterly drawings of the instrument, which performs with dieutmost facility not only all the ingenious experiments of Bou-guer, Lambert, and Leslie, but many others, equally satisfac-tory, may be found in Count Rumford’s work* from whichthese data have been copied.

"I hus far we have examined the two imponderable substancesdiffused through space, namely caloric and light. They have beenconsidered by different philosophers as substances whose parti-cles mutually repel each other, for they are never found coher-ing together.

They agree in many of their properties ; they radiate fromhot or luminous bodies with inconceivable velocity; they aretransmitted, reflected, and refracted by different substances;while they have the property of combining with others.

Caloric we have seen is the cause of the sensadon of heat,it produces expansion, is die cause of fluidity, Ac.

Light on the other hand, produces the sensation of vision,and is the cause of colours. Both substances do not affect theponderability of bodies, all attempts to weigh them have hithertobeen in vain; they have dierefore been called imponderablesubstances, if they are absolutely imponderous is left to thedetermination of the more successful labours of future enquirers.

Rumford’s Philosophical Papers, 1802, p. 270.